chore: refactor tool handling (#4510)

# Tool System Refactor

- Centralizes tool definitions and execution in `core/src/tools/*`:
specs (`spec.rs`), handlers (`handlers/*`), router (`router.rs`),
registry/dispatch (`registry.rs`), and shared context (`context.rs`).
One registry now builds the model-visible tool list and binds handlers.
- Router converts model responses to tool calls; Registry dispatches
with consistent telemetry via `codex-rs/otel` and unified error
handling. Function, Local Shell, MCP, and experimental `unified_exec`
all flow through this path; legacy shell aliases still work.
- Rationale: reduce per‑tool boilerplate, keep spec/handler in sync, and
make adding tools predictable and testable.

Example: `read_file`
- Spec: `core/src/tools/spec.rs` (see `create_read_file_tool`,
registered by `build_specs`).
- Handler: `core/src/tools/handlers/read_file.rs` (absolute `file_path`,
1‑indexed `offset`, `limit`, `L#: ` prefixes, safe truncation).
- E2E test: `core/tests/suite/read_file.rs` validates the tool returns
the requested lines.

## Next steps:
- Decompose `handle_container_exec_with_params` 
- Add parallel tool calls

codex-rs/core/src/client_common.rs

@@ -1,6 +1,6 @@
+use crate::client_common::tools::ToolSpec;
 use crate::error::Result;
 use crate::model_family::ModelFamily;
-use crate::openai_tools::OpenAiTool;
 use crate::protocol::RateLimitSnapshot;
 use crate::protocol::TokenUsage;
 use codex_apply_patch::APPLY_PATCH_TOOL_INSTRUCTIONS;
@@ -29,7 +29,7 @@ pub struct Prompt {
 
     /// Tools available to the model, including additional tools sourced from
     /// external MCP servers.
-    pub(crate) tools: Vec<OpenAiTool>,
+    pub(crate) tools: Vec<ToolSpec>,
 
     /// Optional override for the built-in BASE_INSTRUCTIONS.
     pub base_instructions_override: Option<String>,
@@ -49,8 +49,8 @@ impl Prompt {
         // AND
         // - there is no apply_patch tool present
         let is_apply_patch_tool_present = self.tools.iter().any(|tool| match tool {
-            OpenAiTool::Function(f) => f.name == "apply_patch",
-            OpenAiTool::Freeform(f) => f.name == "apply_patch",
+            ToolSpec::Function(f) => f.name == "apply_patch",
+            ToolSpec::Freeform(f) => f.name == "apply_patch",
             _ => false,
         });
         if self.base_instructions_override.is_none()
@@ -160,6 +160,54 @@ pub(crate) struct ResponsesApiRequest<'a> {
     pub(crate) text: Option<TextControls>,
 }
 
+pub(crate) mod tools {
+    use crate::openai_tools::JsonSchema;
+    use serde::Deserialize;
+    use serde::Serialize;
+
+    /// When serialized as JSON, this produces a valid "Tool" in the OpenAI
+    /// Responses API.
+    #[derive(Debug, Clone, Serialize, PartialEq)]
+    #[serde(tag = "type")]
+    pub(crate) enum ToolSpec {
+        #[serde(rename = "function")]
+        Function(ResponsesApiTool),
+        #[serde(rename = "local_shell")]
+        LocalShell {},
+        // TODO: Understand why we get an error on web_search although the API docs say it's supported.
+        // https://platform.openai.com/docs/guides/tools-web-search?api-mode=responses#:~:text=%7B%20type%3A%20%22web_search%22%20%7D%2C
+        #[serde(rename = "web_search")]
+        WebSearch {},
+        #[serde(rename = "custom")]
+        Freeform(FreeformTool),
+    }
+
+    #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+    pub struct FreeformTool {
+        pub(crate) name: String,
+        pub(crate) description: String,
+        pub(crate) format: FreeformToolFormat,
+    }
+
+    #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+    pub struct FreeformToolFormat {
+        pub(crate) r#type: String,
+        pub(crate) syntax: String,
+        pub(crate) definition: String,
+    }
+
+    #[derive(Debug, Clone, Serialize, PartialEq)]
+    pub struct ResponsesApiTool {
+        pub(crate) name: String,
+        pub(crate) description: String,
+        /// TODO: Validation. When strict is set to true, the JSON schema,
+        /// `required` and `additional_properties` must be present. All fields in
+        /// `properties` must be present in `required`.
+        pub(crate) strict: bool,
+        pub(crate) parameters: JsonSchema,
+    }
+}
+
 pub(crate) fn create_reasoning_param_for_request(
     model_family: &ModelFamily,
     effort: Option<ReasoningEffortConfig>,