codex-rs: fix thread resume rejoin semantics (#11756)

## Summary
- always rejoin an in-memory running thread on `thread/resume`, even
when overrides are present
- reject `thread/resume` when `history` is provided for a running thread
- reject `thread/resume` when `path` mismatches the running thread
rollout path
- warn (but do not fail) on override mismatches for running threads
- add more `thread_resume` integration tests and fixes; including
restart-based resume-with-overrides coverage

## Validation
- `just fmt`
- `cargo test -p codex-app-server --test all thread_resume`
- manual test with app-server-test-client
https://github.com/openai/codex/pull/11755
- manual test both stdio and websocket in app

codex-rs/app-server-test-client/README.md

@@ -17,3 +17,33 @@ cargo run -p codex-app-server-test-client -- \
 # 3) Call app-server (defaults to ws://127.0.0.1:4222)
 cargo run -p codex-app-server-test-client -- model-list
 ```
+
+## Testing Thread Rejoin Behavior
+
+Build and start an app server using commands above. The app-server log is written to `/tmp/codex-app-server-test-client/app-server.log`
+
+### 1) Get a thread id
+
+Create at least one thread, then list threads:
+
+```bash
+cargo run -p codex-app-server-test-client -- send-message-v2 "seed thread for rejoin test"
+cargo run -p codex-app-server-test-client -- thread-list --limit 5
+```
+
+Copy a thread id from the `thread-list` output.
+
+### 2) Rejoin while a turn is in progress (two terminals)
+
+Terminal A:
+
+```bash
+cargo run --bin codex-app-server-test-client -- \
+  resume-message-v2 <THREAD_ID> "respond with thorough docs on the rust core"
+```
+
+Terminal B (while Terminal A is still streaming):
+
+```bash
+cargo run --bin codex-app-server-test-client -- thread-resume <THREAD_ID>
+```