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/src/lib.rs

@@ -275,6 +275,7 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         sandbox_mode,
         cwd: resolved_cwd,
         model_provider: model_provider.clone(),
+        service_tier: None,
         codex_linux_sandbox_exe: arg0_paths.codex_linux_sandbox_exe.clone(),
         main_execve_wrapper_exe: arg0_paths.main_execve_wrapper_exe.clone(),
         js_repl_node_path: None,