Finish moving codex exec to app-server (#15424)

This PR completes the conversion of non-interactive `codex exec` to use
app server rather than directly using core events and methods.

### Summary
- move `codex-exec` off exec-owned `AuthManager` and `ThreadManager`
state
- route exec bootstrap, resume, and auth refresh through existing
app-server paths
- replace legacy `codex/event/*` decoding in exec with typed app-server
notification handling
- update human and JSONL exec output adapters to translate existing
app-server notifications only
- clean up "app server client" layer by eliminating support for legacy
notifications; this is no longer needed
- remove exposure of `authManager` and `threadManager` from "app server
client" layer

### Testing
- `exec` has pretty extensive unit and integration tests already, and
these all pass
- In addition, I asked Codex to put together a comprehensive manual set
of tests to cover all of the `codex exec` functionality (including
command-line options), and it successfully generated and ran these tests

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

@@ -239,8 +239,6 @@ fn build_test_processor(
         cli_overrides: Vec::new(),
         loader_overrides: LoaderOverrides::default(),
         cloud_requirements: CloudRequirementsLoader::default(),
-        auth_manager: None,
-        thread_manager: None,
         feedback: CodexFeedback::new(),
         log_db: None,
         config_warnings: Vec::new(),