tools: remove is_mutating dispatch gating (#22382)

## Why

Tool dispatch had two serialization mechanisms:

- `supports_parallel_tool_calls` decides whether a tool participates in
the shared parallel-execution lock.
- `is_mutating` separately gated some calls inside dispatch.

That second hook no longer carried its weight. The remaining
parallel-support flag is already the per-tool concurrency policy, so
keeping a second mutating gate made dispatch harder to follow and left
behind extra session plumbing that only existed for that path.

## What changed

- Removed `is_mutating` from tool handlers and deleted the
`tool_call_gate` path that existed only to support it.
- Simplified dispatch and routing to rely on the existing per-tool
`supports_parallel_tool_calls` boolean.
- Dropped the now-unused handler overrides and related session/test
scaffolding.
- Kept the router/parallel tests focused on the surviving per-tool
behavior.
- Removed the unused `codex-utils-readiness` dependency from
`codex-core` as a follow-up fix for `cargo shear`.

## Testing

- `cargo test -p codex-core
parallel_support_does_not_match_namespaced_local_tool_names`
- `cargo test -p codex-core mcp_parallel_support_uses_handler_data`
- `cargo test -p codex-core
tools_without_handlers_do_not_support_parallel`

codex-rs/core/src/session/mod.rs

@@ -358,7 +358,6 @@ use codex_tools::ToolEnvironmentMode;
 use codex_tools::ToolsConfig;
 use codex_tools::ToolsConfigParams;
 use codex_utils_absolute_path::AbsolutePathBuf;
-use codex_utils_readiness::ReadinessFlag;
 #[cfg(test)]
 use codex_utils_stream_parser::ProposedPlanSegment;
 

codex-rs/core/src/session/review.rs

@@ -146,7 +146,6 @@ pub(super) async fn spawn_review_thread(
         final_output_json_schema: None,
         codex_self_exe: parent_turn_context.codex_self_exe.clone(),
         codex_linux_sandbox_exe: parent_turn_context.codex_linux_sandbox_exe.clone(),
-        tool_call_gate: Arc::new(ReadinessFlag::new()),
         dynamic_tools: parent_turn_context.dynamic_tools.clone(),
         truncation_policy: model_info.truncation_policy.into(),
         turn_metadata_state,

codex-rs/core/src/session/tests.rs

@@ -2837,10 +2837,6 @@ async fn turn_context_with_model_updates_model_fields() {
         updated.truncation_policy,
         expected_model_info.truncation_policy.into()
     );
-    assert!(!Arc::ptr_eq(
-        &updated.tool_call_gate,
-        &turn_context.tool_call_gate
-    ));
 }
 
 #[test]

codex-rs/core/src/session/turn_context.rs

@@ -89,7 +89,6 @@ pub struct TurnContext {
     pub(crate) final_output_json_schema: Option<Value>,
     pub(crate) codex_self_exe: Option<PathBuf>,
     pub(crate) codex_linux_sandbox_exe: Option<PathBuf>,
-    pub(crate) tool_call_gate: Arc<ReadinessFlag>,
     pub(crate) truncation_policy: TruncationPolicy,
     pub(crate) dynamic_tools: Vec<DynamicToolSpec>,
     pub(crate) turn_metadata_state: Arc<TurnMetadataState>,
@@ -272,7 +271,6 @@ impl TurnContext {
             final_output_json_schema: self.final_output_json_schema.clone(),
             codex_self_exe: self.codex_self_exe.clone(),
             codex_linux_sandbox_exe: self.codex_linux_sandbox_exe.clone(),
-            tool_call_gate: Arc::new(ReadinessFlag::new()),
             truncation_policy,
             dynamic_tools: self.dynamic_tools.clone(),
             turn_metadata_state: self.turn_metadata_state.clone(),
@@ -571,7 +569,6 @@ impl Session {
             final_output_json_schema: None,
             codex_self_exe: per_turn_config.codex_self_exe.clone(),
             codex_linux_sandbox_exe: per_turn_config.codex_linux_sandbox_exe.clone(),
-            tool_call_gate: Arc::new(ReadinessFlag::new()),
             truncation_policy: model_info.truncation_policy.into(),
             dynamic_tools: session_configuration.dynamic_tools.clone(),
             turn_metadata_state,