🤖 fix: prevent queued message amnesia by flushing partial before tool-call-end

When a queued message is sent after tool-call-end, the model needs to see
the complete assistant response including tool results. Previously, there
was a race condition where tool-call-end was emitted before the partial
file was written, causing commitToHistory to read stale data.

The fix ensures partial.json is flushed to disk BEFORE emitting the
tool-call-end event, so listeners (like sendQueuedMessages) see the
complete tool result when they read via commitToHistory.

Changes:
- Make completeToolCall async
- Await flushPartialWrite before emitting tool-call-end
- Update callers to await the async method

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Author: ethanndickson