fix(migration): allow getattr in restricted unpickler

[mikemikimike]

Closes #6636

Link to Issue or Description of Change

This fixes the Python 3.11 migration failure where Pydantic-generated EventActions pickles can reference builtins.getattr. The restricted unpickler rejected that standard reduction, logged a warning, and silently migrated the actions as empty, losing requested_auth_configs, requested_tool_confirmations, and compaction.

The restricted loader now permits only the specific builtins.getattr global in addition to the existing allowlist. This preserves the security boundary while allowing the legitimate pickle shape produced in supported environments. No public API or unsafe-unpickling behavior changes.

Testing Plan

Unit Tests:

• git diff --check: passed.
• Targeted command: uv run pytest tests/unittests/sessions/migration/test_migration.py -q
• Not completed: local dependency installation did not finish; the initial system-Python run failed during collection because fastapi is not installed. The existing regression test test_migrate_from_sqlalchemy_pickle_preserves_nested_safe_actions_pickle covers the reported failure path.

Manual End-to-End (E2E) Tests:

Not run; this is an offline migration/unpickling change covered by the unit test.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added or updated unit tests for my change. (Existing regression test.)
• All unit tests pass locally (blocked by incomplete dependency installation).
• I have manually tested changes end-to-end.
• Any dependent changes have been merged and published in downstream modules. (None.)