Add thread/settings/update app-server API (#23502)

## Why

App-server clients need a way to update a thread's next-turn settings
without starting a turn, adding transcript content, or waiting for turn
lifecycle events. This gives settings UI a direct path for durable
thread settings while clients observe the eventual effective state
through a notification.

This is a simplified rework of PR
https://github.com/openai/codex/pull/22509. In particular, it changes
the `thread/settings/update` api to return immediately rather than
waiting and returning the effective (updated) thread settings. This
makes the new api consistent with `turn/start` and greatly reduces the
complexity of the implementation relative to the earlier attempt.

## What Changed

- Adds experimental `thread/settings/update` with partial-update request
fields and an empty acknowledgment response.
- Adds experimental `thread/settings/updated`, carrying full effective
`ThreadSettings` and scoped by `threadId` to subscribed clients for the
affected thread.
- Shares durable settings validation with `turn/start`, including
`sandboxPolicy` plus `permissions` rejection and `serviceTier: null`
clearing.
- Emits the same settings notification when `turn/start` overrides
change the stored effective thread settings.
- Regenerates app-server protocol schema fixtures and updates
`app-server/README.md`.

codex-rs/app-server/src/message_processor.rs

@@ -1040,6 +1040,11 @@ impl MessageProcessor {
             ClientRequest::ThreadMetadataUpdate { params, .. } => {
                 self.thread_processor.thread_metadata_update(params).await
             }
+            ClientRequest::ThreadSettingsUpdate { params, .. } => {
+                self.turn_processor
+                    .thread_settings_update(&request_id, params)
+                    .await
+            }
             ClientRequest::ThreadMemoryModeSet { params, .. } => {
                 self.thread_processor.thread_memory_mode_set(params).await
             }