## Summary
This PR fixes a UI/streaming race when nudged or steer-enabled messages
are queued during an active Plan stream.
Previously, `submit_user_message_with_mode` switched collaboration mode
immediately (via `set_collaboration_mask`) even when the message was
queued. If that happened mid-Plan stream, `active_mode_kind` could flip
away from Plan before the turn finished, causing subsequent
`on_plan_delta` updates to be ignored in the UI.
Now, mode switching is deferred until the queued message is actually
submitted.
## What changed
- Added a per-message deferred mode override on `UserMessage`:
- `collaboration_mode_override: Option<CollaborationModeMask>`
- Updated `submit_user_message_with_mode` to:
- create a `UserMessage` carrying the mode override
- queue or submit that message without mutating global mode immediately
- Updated `submit_user_message` to:
- apply `collaboration_mode_override` just before constructing/sending
`Op::UserTurn`
- Kept queueing condition scoped to active Plan stream rendering:
- queue only while plan output is actively streaming in TUI
(`plan_stream_controller.is_some()`)
## Why
This preserves Plan mode for the remainder of the in-flight Plan turn,
so streamed plan deltas continue rendering correctly, while still
ensuring the follow-up queued message is sent with the intended
collaboration mode.
## Behavior after this change
- If a nudged/steer submission happens while Plan output is actively
streaming:
- message is queued
- UI stays in Plan mode for the running turn
- once dequeued/submitted, mode override is applied and the message is
sent in the intended mode
- If no Plan stream is active:
- submission proceeds immediately and mode override is applied as before
## Tests
Added/updated coverage in `tui/src/chatwidget/tests.rs`:
- `submit_user_message_with_mode_queues_while_plan_stream_is_active`
- asserts mode remains Plan while queued
- asserts mode switches to Code when queued message is actually
submitted
- `submit_user_message_with_mode_submits_when_plan_stream_is_not_active`
- `steer_enter_queues_while_plan_stream_is_active`
- `steer_enter_submits_when_plan_stream_is_not_active`
Also updated existing `UserMessage { ... }` test fixtures to include the
new field.
## Codex author
`codex fork 019c1047-d5d5-7c92-a357-6009604dc7e8`
b7ecd166a6