[5 of 7] Replace OverrideTurnContext with ThreadSettings (#22508)

**Stack position:** [5 of 7]

## Summary

This PR adds `Op::ThreadSettings`, a queued settings-only update
mechanism for changing stored thread settings without starting a new
turn. It also removes the legacy `Op::OverrideTurnContext` in the same
layer, so reviewers can see the replacement and deletion together.

## Changes

- Add `Op::ThreadSettings` for settings-only queued updates.
- Emit `ThreadSettingsApplied` with the effective thread settings
snapshot after core applies an update.
- Route settings-only updates through the same submission queue as user
input.
- Migrate remaining `OverrideTurnContext` tests and callers to the
queued `Op::ThreadSettings` path.
- Delete `Op::OverrideTurnContext` from the core protocol and submission
loop.

This stack addresses #20656 and #22090.

## Stack

1. [1 of 7] [Add thread settings to
UserInput](https://github.com/openai/codex/pull/23080)
2. [2 of 7] [Remove
UserInputWithTurnContext](https://github.com/openai/codex/pull/23081)
3. [3 of 7] [Remove
UserTurn](https://github.com/openai/codex/pull/23075)
4. [4 of 7] [Placeholder for OverrideTurnContext
cleanup](https://github.com/openai/codex/pull/23087)
5. [5 of 7] [Replace OverrideTurnContext with
ThreadSettings](https://github.com/openai/codex/pull/22508) (this PR)
6. [6 of 7] [Add app-server thread settings
API](https://github.com/openai/codex/pull/22509)
7. [7 of 7] [Sync TUI thread
settings](https://github.com/openai/codex/pull/22510)

codex-rs/app-server/src/request_processors/thread_processor_tests.rs

@@ -62,6 +62,9 @@ mod thread_processor_behavior_tests {
     use codex_model_provider_info::ModelProviderInfo;
     use codex_model_provider_info::WireApi;
     use codex_protocol::ThreadId;
+    use codex_protocol::config_types::CollaborationMode;
+    use codex_protocol::config_types::ModeKind;
+    use codex_protocol::config_types::Settings;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_DANGER_FULL_ACCESS;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_READ_ONLY;
     use codex_protocol::models::BUILT_IN_PERMISSION_PROFILE_WORKSPACE;
@@ -662,7 +665,16 @@ mod thread_processor_behavior_tests {
             profile_workspace_roots: Vec::new(),
             ephemeral: false,
             reasoning_effort: None,
+            reasoning_summary: None,
             personality: None,
+            collaboration_mode: CollaborationMode {
+                mode: ModeKind::Default,
+                settings: Settings {
+                    model: "gpt-5".to_string(),
+                    reasoning_effort: None,
+                    developer_instructions: None,
+                },
+            },
             session_source: SessionSource::Cli,
             thread_source: None,
         };

codex-rs/app-server/src/request_processors/turn_processor.rs

@@ -479,7 +479,7 @@ impl TurnRequestProcessor {
         // still queued together with the input below to preserve submission order.
         if has_any_overrides {
             thread
-                .validate_thread_settings_overrides(CodexThreadSettingsOverrides {
+                .preview_thread_settings_overrides(CodexThreadSettingsOverrides {
                     cwd: cwd.clone(),
                     workspace_roots: runtime_workspace_roots.clone(),
                     approval_policy,