fix: reject MQTT WebSocket text frames

[OllieinCanada]

Fixes #138

Summary

Reject inbound WebSocket text data frames instead of forwarding their UTF-8 bytes to the MQTT decoder.

MQTT 5 requires MQTT control packets to use WebSocket binary data frames. The adapter now returns a clear InvalidData transport error for any text frame, which takes the normal connection teardown path. Binary frames and Ping, Pong, and Close handling are unchanged.

Tests

Added deterministic localhost-only WebSocket tests that verify:

• binary MQTT packets still decode after a control frame
• valid-looking MQTT bytes carried in a text frame are rejected
• empty text frames are also rejected

The main regression failed before the production change because the text frame decoded as Ok(RawPacket { first_byte: 48, ... }).

Validation after the fix:

• cargo test --lib --features websockets io::tokio_ws::tests (3 passed)
• make test (default, integration, and WebSocket feature sets passed)
• cargo fmt --all -- --check
• Clippy for both repository feature matrices with warnings denied
• git diff --check

No external broker or network service is required by the new tests.

[OllieinCanada]

@microsoft-github-policy-service agree