Refactor the Meshtastic TCP pub/sub example to ensure proper resource cleanup and clearer exception handling.

[dim5x]

What changed:

• Wrapped the TCP interface lifecycle in a try / finally block to guarantee that TCPInterface.close() is always called.
• Initialized iface to None to safely handle failures during interface creation.
• Replaced sys.exit(1) with re-raising the exception to preserve the original traceback.
• Simplified imports by importing TCPInterface directly.
• Kept the example behavior and public API unchanged.

Why:

• The original example never reached iface.close() due to the infinite loop and could leave the TCP connection open when interrupted or when an exception occurred.
• Using finally ensures deterministic cleanup and makes the example safer and easier to reason about, especially for users copying it into their own scripts.
• Re-raising the exception provides better diagnostics while still allowing finally to run.

Behavior impact:

• No functional changes to the demo logic.
• Improved reliability and correctness on errors or interruptions.
• More idiomatic Python resource handling.

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