[gauravch-code]

Description

Keep retired sessions from becoming usable again when mark_good() lowers their error score. A session explicitly retired after a blocked request should not be handed out again by the pool, including after saving and restoring the pool.

• Track retirement independently of the error score and persist it in SessionModel.
• Default the new flag to False when loading older saved state.
• Make repeated retire() calls leave the error score unchanged.
• Document the behavior change in the v2 migration guide.

Issues

Closes #2312

Testing

• Reproduced the original behavior on unchanged v2: a fresh session becomes usable after retire() followed by mark_good() (error score 3.0 -> 2.5). Two regression tests failed before the fix.
• Added six tests covering terminal retirement, JSON round trips, older saved state, repeated retirement, pool lookups, and pool persistence/reopening.
• uv run pytest tests/unit/sessions tests/unit/crawlers/_basic -q --timeout=90: 193 passed, 1 existing skip on Windows / Python 3.12.13.
• Repository-wide Ruff lint and formatting checks passed.
• Type checks passed for the changed modules and tests. The full repository check passed with --python-platform linux.
• Both pre-commit hooks passed using a temporary, untracked ty configuration matching the repository settings with the Linux target. That configuration was removed and is not part of this PR.

The default Windows repository-wide type check reports three errors in the unchanged tests/unit/_utils/test_system.py involving ForkContext, ForkServerContext, and BaseContext.Process. Full unit and platform end-to-end suites were not run.

Checklist

• CI passed