fix: prevent head-of-line blocking in OutgoingMessageQueue

[davidrimshnick]

Summary

• Changes processQueueInternal() to skip unreleased items instead of stopping entirely when encountering one at the head of the queue
• This prevents immediately-ready messages (e.g., background task results, sidechain responses) from being blocked behind delayed tool call messages
• The "one behind" symptom occurs because tool call messages have a 250ms delay, and any messages enqueued during that delay are blocked until it expires

Fixes #639

Test plan

• Added 8 unit tests covering ordering, delay, head-of-line bypass, releaseToolCall, flush, and interleaved messages
• Core test: "should NOT block released messages behind unreleased ones" verifies the fix
• Verify in production that background task responses appear immediately instead of one step behind

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