fix(gateway): prioritize user interrupts over internal events

[rungmc357]

Summary

When a gateway session is busy, synthetic internal=True completion events intentionally queue without interrupting the active turn. If one reaches the pending head before a real user follow-up, the user message can still abort the model call but remain behind that internal event in the FIFO. The recursive next turn then handles the synthetic notification first, delaying the user's steering despite the interrupt acknowledgement.

This change gives real user events priority over queued internal events while preserving arrival order within each class. When the bounded queue is full, it prefers dropping a synthetic notification rather than live operator input.

Related follow-ups to #49738 (internal completions do not interrupt busy sessions) and #49776 (busy-session FIFO queueing). I searched existing issues and PRs and did not find an equivalent fix.

Reproduction

1. Start a gateway turn with busy_input_mode: interrupt.
2. While it is running, enqueue a background/delegation completion (internal=True).
3. Send a real user follow-up before the active model call completes.
4. The model call is interrupted, but plain FIFO dequeues the older internal completion first instead of the user message.

Changes

• Reorder a newly queued real user event ahead of pending internal events.
• Preserve user-to-user and internal-to-internal ordering.
• Preserve existing media merge behavior and the queue depth cap.
• Add a regression test covering an internal completion already occupying the head slot when a user interrupt arrives.

Test plan

• scripts/run_tests.sh tests/gateway/test_internal_event_never_interrupts_busy_session.py tests/gateway/test_queue_consumption.py tests/gateway/test_queue_command.py tests/gateway/test_busy_session_ack.py tests/gateway/test_session_race_guard.py
  • 67 passed, 0 failed
• uv run --extra dev ruff check .
  • passed
• python3 scripts/check-windows-footguns.py --all
  • passed (753 files scanned)
• git diff --check origin/main...HEAD
  • passed

Risk and rollback

Risk is limited to ordering in a busy session when real and internal events coexist in the same pending queue. Media-only merging, all-user FIFO ordering, and all-internal FIFO ordering are unchanged. The focused queue tests cover the surrounding interrupt and dequeue paths.

Rollback is a single-commit revert; no configuration, migration, or persistent data changes are involved.

[rungmc357]

Rebased onto current main and added focused regression coverage for the busy-session priority invariant.

The new tests exercise the real queue helpers and verify:

• user input is promoted ahead of queued internal events
• FIFO is preserved within user and internal event classes
• a full queue drops or evicts synthetic work before live user steering
• drain and head-promotion order remain correct
• photo/media bursts still merge into the user-owned head slot rather than splitting around internal overflow

The media-burst test fails against the prior production commit and passes with the included head-merge fix, so the production delta is required rather than test-only churn.

Verification on current main:

• 30 focused and sibling tests passed
• Ruff passed
• git diff --check passed