fix(slack-bot): prevent posting to main channel when thread message is deleted

[kevkantes]

Description

When a user posts a message and deletes it before the Slack bot processes the event, the bot would post its response to the main channel instead of a thread. This happened because Slack's chat_postMessage silently ignores an invalid thread_ts and falls back to posting in the channel.

This PR adds a multi-layer guard:

1. Pre-processing check (verify_thread_exists in utils.py): Before processing any event, calls conversations_replies to confirm the parent message still exists. If deleted, the event is dropped. Applied to all 4 entry points: handle_mention, handle_qanda_message, handle_dm_message, and handle_ai_alert_processing.

2. Mid-stream abort (thread_deleted flag in ai.py): If the message is deleted during A2A streaming (race condition), startStream fails with invalid_thread_ts. Instead of logging 30+ warnings and falling through to chat_postMessage, we set a thread_deleted flag, break out of the stream loop, and drop the response.

Additionally migrates the slack bot from requirements.txt + pip to pyproject.toml + uv, matching the rest of the project's dependency management:

• New pyproject.toml with explicit PyPI index and unittest dependency group
• Updated Dockerfile.slack-bot to use uv sync --locked
• Added command override in docker-compose.dev.yaml to handle volume-mount dev mode
• Removed requirements.txt

Type of Change

• Bugfix
• Refactor

Checklist

• I have read the contributing guidelines
• I have verified this change is not present in other open pull requests
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass (70/70)

[github-actions]

✅ No proprietary content detected. This PR is clear for review!

[github-actions]

📊 Test Coverage Report

Main Tests Coverage

Metric	Coverage	Details
Lines	39.8%	7495/18845 lines
Branches	0.0%	0/0 branches

📁 Coverage Artifacts

• Main tests: coverage-reports-main artifact
• RAG tests: coverage-reports-rag artifact (not available)
• Download artifacts to view detailed HTML coverage reports

[github-actions]

✅ No proprietary content detected. This PR is clear for review!

[github-actions]

📊 Test Coverage Report

Main Tests Coverage

Metric	Coverage	Details
Lines	39.8%	7495/18845 lines
Branches	0.0%	0/0 branches

📁 Coverage Artifacts

• Main tests: coverage-reports-main artifact
• RAG tests: coverage-reports-rag artifact (not available)
• Download artifacts to view detailed HTML coverage reports