Fix: Split long messages at newlines for Slack API 3000 char limit

[remote-swe-user]

Problem

The sendMessageToUser tool encounters Slack API errors when messages exceed 3000 characters.

Solution

This PR implements message splitting functionality in the sendMessageToSlack function to handle long messages:

Changes Made

• Added splitMessageByNewlines function: Splits messages at newline boundaries when they exceed the 3000 character limit
• Modified sendMessageToSlack function: Now sends multiple messages sequentially when content is too long
• Smart splitting logic:
  • Prioritizes splitting at newline characters within the 3000 character limit
  • Falls back to hard cut if no newlines are found (safety measure)
  • Preserves message formatting and readability

Implementation Details

1. Character limit check: Messages ≤ 3000 chars are sent as before (no change in behavior)
2. Newline-based splitting: For longer messages, finds the last newline within 3000 chars
3. Sequential sending: Each chunk is sent as a separate Slack message in the same thread
4. Error prevention: Eliminates the 3000 character limit errors from Slack API

Testing

• ✅ TypeScript compilation passes
• ✅ Prettier formatting applied
• ✅ All existing tests pass
• ✅ No breaking changes to existing functionality

This ensures that long AI responses and code outputs are properly delivered to users without API failures.