[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/app-server/src/request_processors.rs

@@ -259,7 +259,7 @@ use codex_config::ConfigLayerStack;
 use codex_config::loader::project_trust_key;
 use codex_config::types::McpServerTransportConfig;
 use codex_core::CodexThread;
-use codex_core::CodexThreadTurnContextOverrides;
+use codex_core::CodexThreadSettingsOverrides;
 use codex_core::ExternalGoalPreviousStatus;
 use codex_core::ExternalGoalSet;
 use codex_core::ForkSnapshot;

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

@@ -457,7 +457,7 @@ impl TurnRequestProcessor {
                     warning.contains("Configured value for `permission_profile` is disallowed")
                 }) {
                     return Err(invalid_request(format!(
-                        "invalid turn context override: {warning}"
+                        "invalid thread settings override: {warning}"
                     )));
                 }
                 (
@@ -479,7 +479,7 @@ impl TurnRequestProcessor {
         // still queued together with the input below to preserve submission order.
         if has_any_overrides {
             thread
-                .validate_turn_context_overrides(CodexThreadTurnContextOverrides {
+                .validate_thread_settings_overrides(CodexThreadSettingsOverrides {
                     cwd: cwd.clone(),
                     workspace_roots: runtime_workspace_roots.clone(),
                     approval_policy,
@@ -497,7 +497,9 @@ impl TurnRequestProcessor {
                     personality,
                 })
                 .await
-                .map_err(|err| invalid_request(format!("invalid turn context override: {err}")))?;
+                .map_err(|err| {
+                    invalid_request(format!("invalid thread settings override: {err}"))
+                })?;
         }
 
         // Start the turn by submitting the user input. Return its submission id as turn_id.
@@ -529,6 +531,7 @@ impl TurnRequestProcessor {
                 environments: environment_selections,
                 final_output_json_schema: params.output_schema,
                 responsesapi_client_metadata: params.responsesapi_client_metadata,
+                thread_settings: Default::default(),
             }
         };
         let turn_id = self