Remove disconnected drawing board participants

[dajiaohuang]

Disconnected drawing-board clients remain in board['handlers'] because unregister_participant checks the outer board metadata instead of the handler map. Check the handler map before removing the participant.

Fixes #306.

Added tests that import the actual example and instantiate its plugin and handlers on an isolated CherryPy bus. They cover removal without affecting another participant, the connection-close callback, and harmless repeated or unknown unregister calls. No server or monitor thread is started. Mako is included in the test requirements because the example imports it.

Validation: all three regression tests fail against the unchanged upstream implementation and pass with the fix. The complete suite passes on Linux/Python 3.12: 127 passed. The three focused tests also pass on Windows/Python 3.13.

[auvipy]

please add appropriate unit tests for this change

[dajiaohuang]

Added the requested unit tests for the actual drawing-board plugin, including its connection-close callback, preservation of another participant, and repeated/unknown unregister calls. All three fail on the unchanged upstream implementation and pass with this fix. The full Linux/Python 3.12 suite passes: 127 tests.