fix: drop non-UTF-8 cloud mips message instead of crashing mqtt loop

[Heartcoolman]

Problem

Fixes #1749.

MipsCloudClient._on_mips_message decodes every cloud push with a bare payload.decode('utf-8') (miot_mips.py:1104). Some cloud pushes carry payloads that are not valid UTF-8 — on my instance this fired 19 times in 8 days (offending byte always around position 162–190, e.g. 0xaa, 0x80, 0xee), each time raising UnicodeDecodeError inside paho's on_message:

File "/config/custom_components/xiaomi_home/miot/miot_mips.py", line 1104, in _on_mips_message
    payload_str: str = payload.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xee in position 168: invalid continuation byte

The exception aborts paho's read loop (__mqtt_loop_handler re-raises) and the message is lost. When a lost push is a device online/offline state notification, the device's entities stay unavailable in HA indefinitely — in my case a linp.light.lx2bcw was stuck for ~17 hours while working fine in the Mi Home app, and reloading the config entry restored it instantly. Full logs in #1749.

Change

Catch UnicodeDecodeError in MipsCloudClient._on_mips_message, log the topic and the payload hex (so the actual encoding of these messages can be diagnosed from user reports), and skip the message. One malformed push can no longer abort MQTT message processing.

The failing byte offsets (160+ bytes decode fine, then an invalid byte) suggest mostly-JSON text with an embedded non-UTF-8 sequence rather than a packed/TLV message, so no alternative decode path is attempted — the message is dropped defensively.

Testing

• Standalone unit exercise of _on_mips_message (duck-typed client):
  • valid UTF-8 payload → handler dispatched with decoded string (unchanged behavior);
  • non-UTF-8 payload (byte pattern from production logs) → no exception, one log_error, handler not called;
  • no-subscriber early-return path unaffected.
• pylint --rcfile=.pylintrc on the file: 10.00/10 (same as before the change).
• test/test_mips.py requires live cloud credentials/gateway, not run.

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