[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/core/src/codex_thread.rs

@@ -63,7 +63,9 @@ pub struct ThreadConfigSnapshot {
     pub profile_workspace_roots: Vec<AbsolutePathBuf>,
     pub ephemeral: bool,
     pub reasoning_effort: Option<ReasoningEffort>,
+    pub reasoning_summary: Option<ReasoningSummary>,
     pub personality: Option<Personality>,
+    pub collaboration_mode: CollaborationMode,
     pub session_source: SessionSource,
     pub thread_source: Option<ThreadSource>,
 }
@@ -257,11 +259,19 @@ impl CodexThread {
             .await
     }
 
-    /// Validate persistent thread settings overrides without committing them.
-    pub async fn validate_thread_settings_overrides(
+    /// Preview persistent thread settings overrides without committing them.
+    pub async fn preview_thread_settings_overrides(
         &self,
         overrides: CodexThreadSettingsOverrides,
-    ) -> ConstraintResult<()> {
+    ) -> ConstraintResult<ThreadConfigSnapshot> {
+        let updates = self.thread_settings_update(overrides).await;
+        self.codex.session.preview_settings(&updates).await
+    }
+
+    async fn thread_settings_update(
+        &self,
+        overrides: CodexThreadSettingsOverrides,
+    ) -> SessionSettingsUpdate {
         let CodexThreadSettingsOverrides {
             cwd,
             workspace_roots,
@@ -289,7 +299,7 @@ impl CodexThread {
                 .with_updates(model, effort, /*developer_instructions*/ None)
         };
 
-        let updates = SessionSettingsUpdate {
+        SessionSettingsUpdate {
             cwd,
             workspace_roots,
             profile_workspace_roots,
@@ -304,8 +314,7 @@ impl CodexThread {
             service_tier,
             personality,
             ..Default::default()
-        };
-        self.codex.session.validate_settings(&updates).await
+        }
     }
 
     /// Use sparingly: this is intended to be removed soon.