Unified execution (#3288)

## Unified PTY-Based Exec Tool

Note: this requires to have this flag in the config:
`use_experimental_unified_exec_tool=true`

- Adds a PTY-backed interactive exec feature (“unified_exec”) with
session reuse via
  session_id, bounded output (128 KiB), and timeout clamping (≤ 60 s).
- Protocol: introduces ResponseItem::UnifiedExec { session_id,
arguments, timeout_ms }.
- Tools: exposes unified_exec as a function tool (Responses API);
excluded from Chat
  Completions payload while still supported in tool lists.
- Path handling: resolves commands via PATH (or explicit paths), with
UTF‑8/newline‑aware
  truncation (truncate_middle).
- Tests: cover command parsing, path resolution, session
persistence/cleanup, multi‑session
  isolation, timeouts, and truncation behavior.

codex-rs/core/src/config.rs

@@ -172,6 +172,9 @@ pub struct Config {
 
     pub use_experimental_streamable_shell_tool: bool,
 
+    /// If set to `true`, used only the experimental unified exec tool.
+    pub use_experimental_unified_exec_tool: bool,
+
     /// Include the `view_image` tool that lets the agent attach a local image path to context.
     pub include_view_image_tool: bool,
 
@@ -487,6 +490,7 @@ pub struct ConfigToml {
     pub experimental_instructions_file: Option<PathBuf>,
 
     pub experimental_use_exec_command_tool: Option<bool>,
+    pub experimental_use_unified_exec_tool: Option<bool>,
 
     pub projects: Option<HashMap<String, ProjectConfig>>,
 
@@ -837,6 +841,9 @@ impl Config {
             use_experimental_streamable_shell_tool: cfg
                 .experimental_use_exec_command_tool
                 .unwrap_or(false),
+            use_experimental_unified_exec_tool: cfg
+                .experimental_use_unified_exec_tool
+                .unwrap_or(true),
             include_view_image_tool,
             active_profile: active_profile_name,
             disable_paste_burst: cfg.disable_paste_burst.unwrap_or(false),
@@ -1212,6 +1219,7 @@ model_verbosity = "high"
                 tools_web_search_request: false,
                 preferred_auth_method: AuthMode::ChatGPT,
                 use_experimental_streamable_shell_tool: false,
+                use_experimental_unified_exec_tool: true,
                 include_view_image_tool: true,
                 active_profile: Some("o3".to_string()),
                 disable_paste_burst: false,
@@ -1269,6 +1277,7 @@ model_verbosity = "high"
             tools_web_search_request: false,
             preferred_auth_method: AuthMode::ChatGPT,
             use_experimental_streamable_shell_tool: false,
+            use_experimental_unified_exec_tool: true,
             include_view_image_tool: true,
             active_profile: Some("gpt3".to_string()),
             disable_paste_burst: false,
@@ -1341,6 +1350,7 @@ model_verbosity = "high"
             tools_web_search_request: false,
             preferred_auth_method: AuthMode::ChatGPT,
             use_experimental_streamable_shell_tool: false,
+            use_experimental_unified_exec_tool: true,
             include_view_image_tool: true,
             active_profile: Some("zdr".to_string()),
             disable_paste_burst: false,
@@ -1399,6 +1409,7 @@ model_verbosity = "high"
             tools_web_search_request: false,
             preferred_auth_method: AuthMode::ChatGPT,
             use_experimental_streamable_shell_tool: false,
+            use_experimental_unified_exec_tool: true,
             include_view_image_tool: true,
             active_profile: Some("gpt5".to_string()),
             disable_paste_burst: false,