[app-server] feat: v2 Turn APIs (#6216)

Implements:
```
turn/start
turn/interrupt
```

along with their integration tests. These are relatively light wrappers
around the existing core logic, and changes to core logic are minimal.

However, an improvement made for developer ergonomics:
- `turn/start` replaces both `SendUserMessage` (no turn overrides) and
`SendUserTurn` (can override model, approval policy, etc.)

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -1,6 +1,7 @@
-// v2 test suite modules
 mod account;
 mod thread_archive;
 mod thread_list;
 mod thread_resume;
 mod thread_start;
+mod turn_interrupt;
+mod turn_start;