fix(reviewer-bot): unify pr assignment guidance

[PLeVasseur]

Summary

• centralize reviewer assignment side effects in the command path so successful PR reviewer changes consistently post PR guidance for /r? @user, /claim, /pass, and reassignment after /away
• keep assignment failure comments and reviewer state updates aligned across the PR reviewer-changing command family instead of duplicating slightly different logic in each handler
• add regression tests covering explicit assignment, claim, pass, and queue-based assignment guidance behavior on PRs

Testing

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

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