fix(tui_gateway): queue mid-turn prompts instead of dropping them on a busy retry

[OutThisLife]

Summary

Follow-up to #52592. Symptom reported (desktop app): "if I send a msg and then interrupt and send another one, it doesn't just stop and listen with prev context in mind — it'll often just not listen."

The fix is in the shared tui_gateway, so it covers both the desktop app and the TUI.

Root cause

When a turn is in flight, prompt.submit rejected the next message with 4009 "session busy" and left clients to cope. The desktop app turns that into a deadline-bounded busy-retry (withSessionBusyRetry, 6s). But turn teardown can outlive that window: the sequential tool executor only checks the interrupt flag between tools (agent/tool_executor.py), so a slow, non-interruptible tool already in flight (web_search, read_file, an MCP call, a long terminal) keeps running after stop. Teardown > 6s → the retry gives up → the resubmitted message is dropped → "it just doesn't listen".

Fix

Wire the previously-dead display.busy_input_mode config into prompt.submit. Instead of rejecting a mid-turn prompt, queue it to run as the next turn (drained in run()'s tail, ahead of goal/notification follow-ups):

• interrupt (default): interrupt the live turn so it winds down promptly, then run the queued message.
• queue: run it after the current turn finishes (no interrupt).
• steer: inject into the live turn if accepted, else queue.

The queued slot pins the sender's transport (so the drained turn streams to the right client) and losslessly merges a second arrival (mirroring the consecutive-user merge in repair_message_sequence). No client deadline, no dropped sends. Combined with #52592, the resumed turn also keeps full alternation-clean context.

Surfaces / compatibility

• Desktop: this is the path that dropped messages (the 6s retry deadline). The desktop ignores the prompt.submit response body, and {status:"queued"} makes its busy-retry resolve immediately instead of racing the deadline.
• TUI: already queues mid-turn input client-side and drains on the real settle edge (busy → false), not a fixed deadline — so it didn't drop messages, it waited. This change does not conflict or double-run: the TUI doesn't call prompt.submit while busy on the normal path (it intercepts and queues locally), so the server queue isn't even hit there. The one place it optimistically sends (useSubmission.ts race fallback) previously relied on the session busy error to re-queue; now the server queues it once and returns success, so that race window gets strictly more robust. Net: the gateway becomes the single source of truth for both clients.

Test plan

• New tests/test_tui_gateway_queue_on_busy.py (10 tests): enqueue pin/merge; _handle_busy_submit per-mode (interrupt/queue/steer + steer-rejected fallback); _drain_queued_prompt fires/claims-running/restores-transport, no-ops when empty or already-running, and releases running on dispatch failure.
• tests/test_tui_gateway_server.py + tests/tui_gateway/ green (289 passed; 1 pre-existing failure unrelated — test_browser_manage_connect_default_local_reports_launch_hint asserts no Chromium is installed, fails only on machines that have Chrome).

[github-actions]

🔎 Lint report: bb/queue-resubmit-on-busy vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 11270 on HEAD, 11268 on base (🆕 +2)

🆕 New issues (2):

Rule	Count
unresolved-attribute	2

First entries

tests/run_agent/test_credits_notices_toggle.py:76: [unresolved-attribute] unresolved-attribute: Unresolved attribute `_credits_session_start_micros` on type `AIAgent`
run_agent.py:2989: [unresolved-attribute] unresolved-attribute: Object of type `Self@get_credits_spent_micros` has no attribute `_credits_session_start_micros`

✅ Fixed issues (1):

Rule	Count
invalid-assignment	1

First entries

tests/run_agent/test_credits_notices_toggle.py:76: [invalid-assignment] invalid-assignment: Object of type `None` is not assignable to attribute `_credits_session_start_micros` of type `int`

Unchanged: 5941 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.