mDNS: ignore truncated DNS headers before updating discovery state

[SeoFood]

Truncated DNS headers raise struct.error in the discovery protocol handlers. Ignore these malformed replies before changing response counters or per-host discovery state, then continue processing valid responses.

The multicast handler also avoids creating empty host entries for replies with no services or only unrelated services.

Fixes #2703.

Validation on this branch with Python 3.12 in Docker (--network none):

• All 59 mDNS unit and functional tests pass. The 24 truncated-header cases and two filtered-reply cases fail against the base code.
• The full test suite passes: 1,273 passed, 5 skipped.
• Formatting, linting and type checks using the project configuration pass.

The tests send synthetic packets followed by valid replies. No Apple TV or Home Assistant instance was used. The outer receive delegate already catches exceptions; this fixes malformed-packet handling and logging, not a demonstrated process crash.