## 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`
This PR adds a dedicated `turn/steer` API for appending user input to an
in-flight turn.
## Motivation
Currently, steering in the app is implemented by just calling
`turn/start` while a turn is running. This has some really weird quirks:
- Client gets back a new `turn.id`, even though streamed
events/approvals remained tied to the original active turn ID.
- All the various turn-level override params on `turn/start` do not
apply to the "steer", and would only apply to the next real turn.
- There can also be a race condition where the client thinks the turn is
active but the server has already completed it, so there might be bugs
if the client has baked in some client-specific behavior thinking it's a
steer when in fact the server kicked off a new turn. This is
particularly possible when running a client against a remote app-server.
Having a dedicated `turn/steer` API eliminates all those quirks.
`turn/steer` behavior:
- Requires an active turn on threadId. Returns a JSON-RPC error if there
is no active turn.
- If expectedTurnId is provided, it must match the active turn (more
useful when connecting to a remote app-server).
- Does not emit `turn/started`.
- Does not accept turn overrides (`cwd`, `model`, `sandbox`, etc.) or
`outputSchema` to accurately reflect that these are not applied when
steering.
