feat: session password set/clear for password-protected PLCs (#792)

[gijzelaerr]

Summary

• Add set_session_password(password) and clear_session_password() to both snap7.Client (sync) and snap7.AsyncClient (async)
• These send real S7 USERDATA PDUs (function group 5, Security) to unlock/lock password-protected PLCs, replacing the previous no-op stubs in ClientMixin
• Password encoding matches the Sharp7/rs-snap7 wire format: XOR each byte with 0x55, rotate left by 3 bits, padded to 8 bytes

Changes

• snap7/s7protocol.py: Add encode_password(), build_set_session_password_request(), build_clear_session_password_request(), and check_userdata_response() to S7Protocol
• snap7/client.py: Override set_session_password / clear_session_password with real protocol operations (uses _send_receive_with_reconnect for auto-reconnect support)
• snap7/async_client.py: Same async overrides using _send_receive
• snap7/client_base.py: Remove old no-op stubs from ClientMixin (both concrete clients now implement the real protocol)
• tests/test_s7protocol.py: Unit tests for password encoding (known test vectors), PDU structure, and check_userdata_response error handling
• tests/test_async_client.py: Update async tests to verify connection-required behavior

Test plan

• uv run pytest tests/ -x -q -- all 1639 tests pass
• uv run pre-commit run --all-files -- all hooks pass (mypy, ruff, formatting)
• End-to-end test against a password-protected PLC (requires hardware)

Closes #792