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/tests/suite/prompt_caching.rs

@@ -191,7 +191,8 @@ async fn prompt_tools_are_consistent_across_requests() {
     let expected_instructions: &str = include_str!("../../prompt.md");
     // our internal implementation is responsible for keeping tools in sync
     // with the OpenAI schema, so we just verify the tool presence here
-    let expected_tools_names: &[&str] = &["shell", "update_plan", "apply_patch", "view_image"];
+    let expected_tools_names: &[&str] =
+        &["unified_exec", "update_plan", "apply_patch", "view_image"];
     let body0 = requests[0].body_json::<serde_json::Value>().unwrap();
     assert_eq!(
         body0["instructions"],