fix(a2a): restore session before permission resume

[guima-why]

Summary

• restore the persisted session before looking up a permission checkpoint
• avoid creating an empty local session while the shared backup is unavailable
• keep the existing AG-UI restore preflight unchanged

Tests

• uv run pytest -q tests/a2a/test_executor.py
• uv run pytest -q tests/agui/test_persistence.py -k "restart_restores_interrupt_resume_and_cancel_identity or resume_stops_before_get_task_when_session_backup_is_missing"
• uv run ruff check src/iac_code/a2a/executor.py tests/a2a/test_executor.py
• uv run ty check src/