tests: centralize in-flight turn cleanup helper (#12271)

## Why

Several tests intentionally exercise behavior while a turn is still
active. The cleanup sequence for those tests (`turn/interrupt` + waiting
for `codex/event/turn_aborted`) was duplicated across files, which made
the rationale easy to lose and the pattern easy to apply inconsistently.

This change centralizes that cleanup in one place with a single
explanatory doc comment.

## What Changed

### Added shared helper

In `codex-rs/app-server/tests/common/mcp_process.rs`:

- Added `McpProcess::interrupt_turn_and_wait_for_aborted(...)`.
- Added a doc comment explaining why explicit interrupt + terminal wait
is required for tests that intentionally leave a turn in-flight.

### Migrated call sites

Replaced duplicated interrupt/aborted blocks with the helper in:

- `codex-rs/app-server/tests/suite/v2/thread_resume.rs`
  - `thread_resume_rejects_history_when_thread_is_running`
  - `thread_resume_rejects_mismatched_path_when_thread_is_running`
- `codex-rs/app-server/tests/suite/v2/turn_start_zsh_fork.rs`
  - `turn_start_shell_zsh_fork_executes_command_v2`
-
`turn_start_shell_zsh_fork_subcommand_decline_marks_parent_declined_v2`
- `codex-rs/app-server/tests/suite/v2/turn_steer.rs`
  - `turn_steer_returns_active_turn_id`

### Existing cleanup retained

In `codex-rs/app-server/tests/suite/v2/turn_start.rs`:

- `turn_start_accepts_local_image_input` continues to explicitly wait
for `turn/completed` so the turn lifecycle is fully drained before test
exit.

## Verification

- `cargo test -p codex-app-server`

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

@@ -747,7 +747,12 @@ async fn turn_start_accepts_local_image_input() -> Result<()> {
     let TurnStartResponse { turn } = to_response::<TurnStartResponse>(turn_resp)?;
     assert!(!turn.id.is_empty());
 
-    // This test only validates that turn/start responds and returns a turn.
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
     Ok(())
 }