## Summary
- switch a few app-server `turn_start` tests from
`codex/event/task_complete` waits to `turn/completed` waits
- avoid matching unrelated/background `task_complete` events
- keep this flaky test fix separate from the /title feature PR
## Why
On Windows ARM CI, these tests can return early after observing a
generic `codex/event/task_complete` notification from another task. That
can leave the mock Responses server with fewer calls than expected and
fail the test with a wiremock verification mismatch.
Using `turn/completed` matches the app-server turn lifecycle
notification the tests actually care about.
## Validation
- `cargo test -p codex-app-server
turn_start_updates_sandbox_and_cwd_between_turns_v2 -- --nocapture`
- `cargo test -p codex-app-server turn_start_exec_approval_ --
--nocapture`
- `just fmt`
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