feat: add pydanticai guard reset

[wondr-wclabs]

Fixes #233

Summary

• Added TealTigerGuard.reset() for the PydanticAI adapter so session cleanup clears cumulative cost, call count, audit trail, per-tool summaries, and frozen state.
• Added reset coverage for accumulated session state and for clearing a frozen guard.
• Modernized the touched adapter files to satisfy the package ruff config while keeping the change confined to guard.py and test_guard.py.

Validation

• uv run --extra dev pytest tests/test_guard.py -q — 62 passed
• uv run --extra dev ruff check src tests
• uv run --extra dev ruff format --check src tests
• git diff --check

[nagasatish007]

approved