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/exec/tests/event_processor_with_json_output.rs

@@ -93,6 +93,7 @@ fn session_configured_produces_thread_started_event() {
             thread_name: None,
             model: "codex-mini-latest".to_string(),
             model_provider_id: "test-provider".to_string(),
+            service_tier: None,
             approval_policy: AskForApproval::Never,
             sandbox_policy: SandboxPolicy::new_read_only_policy(),
             cwd: PathBuf::from("/home/user/project"),