Fix reviewer-bot observer no-op reconcile handling

[PLeVasseur]

Summary

• make ignored PR comment observer runs emit an explicit no-op marker artifact
• teach trusted reconcile to accept that no-op marker and exit cleanly without mutating state
• prevent intentionally ignored observer events from showing up as red reconcile failures

What Changed

• update the PR comment observer workflow so ignored automation/self-comment cases produce an explicit JSON no-op marker artifact instead of succeeding with no payload
• update reconcile handling to validate and accept that no-op marker contract
• add regression coverage for the no-op payload path

Validation

• uv run python -m pytest .github/reviewer-bot-tests/test_reviewer_bot.py .github/reviewer-bot-tests/test_main.py
• uv run ruff check --fix scripts .github/reviewer-bot-tests/test_reviewer_bot.py .github/reviewer-bot-tests/test_main.py

Notes

• this fix preserves fail-closed behavior for malformed or ambiguous payloads
• it only changes the contract for successful observer no-op cases so they stop producing false-positive reconcile failures

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