fix(desktop): interrupt running turn when user sends a message while busy

[raymondclowe]

Problem

When the agent is busy and the user types a correction + Enter, the desktop composer queues the message but never interrupts the running turn. The agent keeps going for minutes, ignoring the user's input. The queue counter grows ("4 queued", "5 queued"...) and only ONE message drains after the turn finally ends — leaving the rest stranded and often sending the agent off on a tangent based on a single out-of-context message.

The CLI has busy_input_mode: interrupt (the default) which fires agent.interrupt() on Enter. The desktop app had no equivalent — it only queued.

Fix

When the user presses Enter with a payload while busy, the composer now both queues the message AND fires onCancel() to interrupt the running turn. This mirrors:

• The CLI's busy_input_mode: interrupt behaviour
• The existing sendQueuedNow busy-path (line ~1568): promote to head → onCancel() → auto-drain on busy→false

The auto-drain mechanism (shouldAutoDrain in composer-queue.ts) then sends the queued message as soon as the agent reports busy: false.

Changes

• apps/desktop/src/app/chat/composer/index.tsx: In submitDraft(), the busy && payloadPresent branch now calls queueCurrentDraft() + onCancel() instead of just queueCurrentDraft()
• apps/desktop/src/app/chat/composer/enter-submit-dom-race.test.tsx: Updated test harness and assertion to expect onCancel to be called when queueing while busy

Related issue

Closes #51665

Note

This PR addresses issue #1 from the issue (queue-not-interrupt). The WebSocket stall (#2) and reconnect queue recovery (#3) need separate investigation.

[teknium1]

Thanks for isolating the busy-composer behavior. The defect is still present on current main, but this PR needs a small relocation before it can be salvaged.

Problems

• submitDraft was extracted from apps/desktop/src/app/chat/composer/index.tsx into apps/desktop/src/app/chat/composer/hooks/use-composer-submit.ts by 773a3703bfc1f8ff2f3aef40d7a565e7f4fe1404. The live busy/payload branch remains queueCurrentDraft() only at hooks/use-composer-submit.ts:151-152, so the current diff no longer targets the production decision point.
• The proposed onCancel() is unconditional after queueCurrentDraft(). That helper can return false when no active queue key exists or enqueueing fails (hooks/use-composer-queue.ts:164-180), so cancellation should occur only after a successful queue.

Suggested changes

• Apply the guarded queue-then-cancel behavior in hooks/use-composer-submit.ts.
• Update the regression test for the relocated path and cover queue failure without cancellation.

Automated hermes-sweeper review.