fix(reviewer-bot): infer pr observer context correctly

[PLeVasseur]

Summary

• make the PR observer payload builder use a PR-specific classifier that does not depend on IS_PULL_REQUEST being set in the observer workflow environment
• keep the issue-vs-PR split in the main comment router while ensuring trusted-direct same-repo human PR comments produce observer_noop payloads in observer builds
• update the regression test so it proves the trusted-direct observer noop works without ambient PR context env hacks

Testing

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

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