feat(ocpp-server): add error handling, configurable params, async tests, and Python...

feat(ocpp-server): add error handling, configurable params, async tests, and Python 3.14+ compat

- Add centralized error handling in _send_command dispatch for
  TimeoutError, OCPPError, and ConnectionClosed
- Make host/port configurable via --host and --port CLI args
- Make boot notification status configurable via --boot-status
- Make TransactionEvent total cost configurable via --total-cost
- Extract server defaults as module-level constants
- Fix asyncio.iscoroutine() deprecation in timer.py: use
  inspect.iscoroutinefunction() + inspect.isawaitable() for Python 3.14+
- Add proper type annotations and docstrings to timer.py
- Add pytest-asyncio for async handler testing (82 tests total)
- Add async tests for all 15 incoming OCPP handlers
- Add error handling tests for command dispatch layer
- Add tests for configurable boot_status and total_cost
- Update README with new server and charging behavior CLI options