feat: feature flag (#4948)

Add proper feature flag instead of having custom flags for everything.
This is just for experimental/wip part of the code
It can be used through CLI:
```bash
codex --enable unified_exec --disable view_image_tool
```

Or in the `config.toml`
```toml
# Global toggles applied to every profile unless overridden.
[features]
apply_patch_freeform = true
view_image_tool = false
```

Follow-up:
In a following PR, the goal is to have a default have `bundles` of
features that we can associate to a model

codex-rs/core/src/features.rs

@@ -0,0 +1,250 @@
+//! Centralized feature flags and metadata.
+//!
+//! This module defines a small set of toggles that gate experimental and
+//! optional behavior across the codebase. Instead of wiring individual
+//! booleans through multiple types, call sites consult a single `Features`
+//! container attached to `Config`.
+
+use crate::config::ConfigToml;
+use crate::config_profile::ConfigProfile;
+use serde::Deserialize;
+use std::collections::BTreeMap;
+use std::collections::BTreeSet;
+
+mod legacy;
+pub(crate) use legacy::LegacyFeatureToggles;
+
+/// High-level lifecycle stage for a feature.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Stage {
+    Experimental,
+    Beta,
+    Stable,
+    Deprecated,
+    Removed,
+}
+
+/// Unique features toggled via configuration.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Feature {
+    /// Use the single unified PTY-backed exec tool.
+    UnifiedExec,
+    /// Use the streamable exec-command/write-stdin tool pair.
+    StreamableShell,
+    /// Use the official Rust MCP client (rmcp).
+    RmcpClient,
+    /// Include the plan tool.
+    PlanTool,
+    /// Include the freeform apply_patch tool.
+    ApplyPatchFreeform,
+    /// Include the view_image tool.
+    ViewImageTool,
+    /// Allow the model to request web searches.
+    WebSearchRequest,
+}
+
+impl Feature {
+    pub fn key(self) -> &'static str {
+        self.info().key
+    }
+
+    pub fn stage(self) -> Stage {
+        self.info().stage
+    }
+
+    pub fn default_enabled(self) -> bool {
+        self.info().default_enabled
+    }
+
+    fn info(self) -> &'static FeatureSpec {
+        FEATURES
+            .iter()
+            .find(|spec| spec.id == self)
+            .unwrap_or_else(|| unreachable!("missing FeatureSpec for {:?}", self))
+    }
+}
+
+/// Holds the effective set of enabled features.
+#[derive(Debug, Clone, Default, PartialEq)]
+pub struct Features {
+    enabled: BTreeSet<Feature>,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct FeatureOverrides {
+    pub include_plan_tool: Option<bool>,
+    pub include_apply_patch_tool: Option<bool>,
+    pub include_view_image_tool: Option<bool>,
+    pub web_search_request: Option<bool>,
+}
+
+impl FeatureOverrides {
+    fn apply(self, features: &mut Features) {
+        LegacyFeatureToggles {
+            include_plan_tool: self.include_plan_tool,
+            include_apply_patch_tool: self.include_apply_patch_tool,
+            include_view_image_tool: self.include_view_image_tool,
+            tools_web_search: self.web_search_request,
+            ..Default::default()
+        }
+        .apply(features);
+    }
+}
+
+impl Features {
+    /// Starts with built-in defaults.
+    pub fn with_defaults() -> Self {
+        let mut set = BTreeSet::new();
+        for spec in FEATURES {
+            if spec.default_enabled {
+                set.insert(spec.id);
+            }
+        }
+        Self { enabled: set }
+    }
+
+    pub fn enabled(&self, f: Feature) -> bool {
+        self.enabled.contains(&f)
+    }
+
+    pub fn enable(&mut self, f: Feature) {
+        self.enabled.insert(f);
+    }
+
+    pub fn disable(&mut self, f: Feature) {
+        self.enabled.remove(&f);
+    }
+
+    /// Apply a table of key -> bool toggles (e.g. from TOML).
+    pub fn apply_map(&mut self, m: &BTreeMap<String, bool>) {
+        for (k, v) in m {
+            match feature_for_key(k) {
+                Some(feat) => {
+                    if *v {
+                        self.enable(feat);
+                    } else {
+                        self.disable(feat);
+                    }
+                }
+                None => {
+                    tracing::warn!("unknown feature key in config: {k}");
+                }
+            }
+        }
+    }
+
+    pub fn from_config(
+        cfg: &ConfigToml,
+        config_profile: &ConfigProfile,
+        overrides: FeatureOverrides,
+    ) -> Self {
+        let mut features = Features::with_defaults();
+
+        let base_legacy = LegacyFeatureToggles {
+            experimental_use_freeform_apply_patch: cfg.experimental_use_freeform_apply_patch,
+            experimental_use_exec_command_tool: cfg.experimental_use_exec_command_tool,
+            experimental_use_unified_exec_tool: cfg.experimental_use_unified_exec_tool,
+            experimental_use_rmcp_client: cfg.experimental_use_rmcp_client,
+            tools_web_search: cfg.tools.as_ref().and_then(|t| t.web_search),
+            tools_view_image: cfg.tools.as_ref().and_then(|t| t.view_image),
+            ..Default::default()
+        };
+        base_legacy.apply(&mut features);
+
+        if let Some(base_features) = cfg.features.as_ref() {
+            features.apply_map(&base_features.entries);
+        }
+
+        let profile_legacy = LegacyFeatureToggles {
+            include_plan_tool: config_profile.include_plan_tool,
+            include_apply_patch_tool: config_profile.include_apply_patch_tool,
+            include_view_image_tool: config_profile.include_view_image_tool,
+            experimental_use_freeform_apply_patch: config_profile
+                .experimental_use_freeform_apply_patch,
+            experimental_use_exec_command_tool: config_profile.experimental_use_exec_command_tool,
+            experimental_use_unified_exec_tool: config_profile.experimental_use_unified_exec_tool,
+            experimental_use_rmcp_client: config_profile.experimental_use_rmcp_client,
+            tools_web_search: config_profile.tools_web_search,
+            tools_view_image: config_profile.tools_view_image,
+        };
+        profile_legacy.apply(&mut features);
+        if let Some(profile_features) = config_profile.features.as_ref() {
+            features.apply_map(&profile_features.entries);
+        }
+
+        overrides.apply(&mut features);
+
+        features
+    }
+}
+
+/// Keys accepted in `[features]` tables.
+fn feature_for_key(key: &str) -> Option<Feature> {
+    for spec in FEATURES {
+        if spec.key == key {
+            return Some(spec.id);
+        }
+    }
+    legacy::feature_for_key(key)
+}
+
+/// Deserializable features table for TOML.
+#[derive(Deserialize, Debug, Clone, Default, PartialEq)]
+pub struct FeaturesToml {
+    #[serde(flatten)]
+    pub entries: BTreeMap<String, bool>,
+}
+
+/// Single, easy-to-read registry of all feature definitions.
+#[derive(Debug, Clone, Copy)]
+pub struct FeatureSpec {
+    pub id: Feature,
+    pub key: &'static str,
+    pub stage: Stage,
+    pub default_enabled: bool,
+}
+
+pub const FEATURES: &[FeatureSpec] = &[
+    FeatureSpec {
+        id: Feature::UnifiedExec,
+        key: "unified_exec",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
+    FeatureSpec {
+        id: Feature::StreamableShell,
+        key: "streamable_shell",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
+    FeatureSpec {
+        id: Feature::RmcpClient,
+        key: "rmcp_client",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
+    FeatureSpec {
+        id: Feature::PlanTool,
+        key: "plan_tool",
+        stage: Stage::Stable,
+        default_enabled: false,
+    },
+    FeatureSpec {
+        id: Feature::ApplyPatchFreeform,
+        key: "apply_patch_freeform",
+        stage: Stage::Beta,
+        default_enabled: false,
+    },
+    FeatureSpec {
+        id: Feature::ViewImageTool,
+        key: "view_image_tool",
+        stage: Stage::Stable,
+        default_enabled: true,
+    },
+    FeatureSpec {
+        id: Feature::WebSearchRequest,
+        key: "web_search_request",
+        stage: Stage::Stable,
+        default_enabled: false,
+    },
+];