feat: use saveChatCompletion in handleChatStream to match handleChatGenerate (MYC-3962)

[sweetmantech]

Summary

• Replace handleChatCompletion with saveChatCompletion in handleChatStream to match /api/chat/generate behavior
• Remove duplicate room creation and user message persistence (now handled in validateChatRequest -> setupConversation)
• Keep assistant message persistence with proper error handling (non-critical, logs errors but doesn't fail the request)

Test plan

• Unit tests pass for handleChatStream (11 tests)
• New tests added for message persistence in onFinish callback:
  • Calls saveChatCompletion with text from last assistant message
  • Uses responseMessage as fallback when no assistant messages
  • Does not call saveChatCompletion when stream is aborted
  • Logs error but does not throw when saveChatCompletion fails
• All 564 tests pass

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Project	Deployment	Review	Updated (UTC)
recoup-api	Ready	Preview	Jan 19, 2026 6:47pm

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