[1 of 7] Add thread settings to UserInput (#23080)

**Stack position:** [1 of 7]

## Summary

The first three PRs in this stack are a cleanup pass before the actual
thread settings API work.

Today, core has several overlapping "user input" ops: `UserInput`,
`UserInputWithTurnContext`, and `UserTurn`. They differ mostly in how
much next-turn state they carry, which makes the later queued thread
settings update harder to reason about and review.

This PR starts that cleanup by adding the shared
`ThreadSettingsOverrides` payload and allowing `Op::UserInput` to carry
it. Existing variants remain in place here, so this layer is mostly a
behavior-preserving API shape change plus mechanical constructor
updates.

## End State After PR3

By the end of PR3, `Op::UserInput` is the only "user input" core op. It
can carry optional thread settings overrides for callers that need to
update stored defaults with a turn, while callers without updates use
empty settings. `Op::UserInputWithTurnContext` and `Op::UserTurn` are
deleted.

## End State After PR5

By the end of PR5, core will have only two ops for this area:

- `Op::UserInput` for user-input-bearing submissions.
- `Op::ThreadSettings` for settings-only updates.

## Stack

1. [1 of 7] [Add thread settings to
UserInput](https://github.com/openai/codex/pull/23080) (this PR)
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)
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

@@ -80,9 +80,9 @@ impl ThreadConfigSnapshot {
     }
 }
 
-/// Turn context overrides that app-server validates before starting a turn.
+/// Thread settings overrides that app-server validates before starting a turn.
 #[derive(Clone, Default)]
-pub struct CodexThreadTurnContextOverrides {
+pub struct CodexThreadSettingsOverrides {
     pub cwd: Option<PathBuf>,
     pub workspace_roots: Option<Vec<AbsolutePathBuf>>,
     pub profile_workspace_roots: Option<Vec<AbsolutePathBuf>>,
@@ -257,12 +257,12 @@ impl CodexThread {
             .await
     }
 
-    /// Validate persistent turn context overrides without committing them.
-    pub async fn validate_turn_context_overrides(
+    /// Validate persistent thread settings overrides without committing them.
+    pub async fn validate_thread_settings_overrides(
         &self,
-        overrides: CodexThreadTurnContextOverrides,
+        overrides: CodexThreadSettingsOverrides,
     ) -> ConstraintResult<()> {
-        let CodexThreadTurnContextOverrides {
+        let CodexThreadSettingsOverrides {
             cwd,
             workspace_roots,
             profile_workspace_roots,