app-server service tier plumbing (plus some cleanup) (#13334)

followup to https://github.com/openai/codex/pull/13212 to expose fast
tier controls to app server
(majority of this PR is generated schema jsons - actual code is +69 /
-35 and +24 tests )

- add service tier fields to the app-server protocol surfaces used by
thread lifecycle, turn start, config, and session configured events
- thread service tier through the app-server message processor and core
thread config snapshots
- allow runtime config overrides to carry service tier for app-server
callers

cleanup:
- Removing useless "legacy" code supporting "standard" - we moved to
None | "fast", so "standard" is not needed.

codex-rs/app-server-protocol/src/protocol/mod.rs

@@ -3,6 +3,7 @@
 
 pub mod common;
 mod mappers;
+mod serde_helpers;
 pub mod thread_history;
 pub mod v1;
 pub mod v2;