Reject AUTH, server-sent DISCONNECT and payload bytes in UNSUBACK below v5. Publish Will on write failure.

[kareem-wolfssl]

Fixes #580 #581 #582

[Copilot]

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Pull request overview

This PR tightens MQTT protocol-level compliance by rejecting v5-only packets (AUTH, server-sent DISCONNECT) on v3.1.1 sessions, enforcing exact UNSUBACK length rules for v3.1.1, and ensuring broker-side Will publishing occurs on direct-response write failures.

Changes:

• Enforce v3.1.1 UNSUBACK Remaining Length == 2, while allowing v5 UNSUBACK bodies.
• Reject AUTH (type 15) and server-sent DISCONNECT on protocol level 4 sessions (read + write paths).
• Normalize broker direct-response write failures as fatal so connections close and Wills publish; add tests for these scenarios.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/test_mqtt_packet.c	Adds UNSUBACK decoding tests for v3.1.1 exact-length and v5 longer-body acceptance.
tests/test_mqtt_client.c	Adds client wait/send tests rejecting v5-only packet types on v3.1.1 sessions.
tests/test_broker_connect.c	Adds broker integration tests verifying Will publication on response write failures.
src/mqtt_packet.c	Implements protocol-level-sensitive UNSUBACK Remaining Length validation.
src/mqtt_client.c	Rejects AUTH and server-sent DISCONNECT on v3.1.1 sessions; blocks AUTH send on non-v5 sessions.
src/mqtt_broker.c	Adds write-result normalization for direct responses and treats write failures as fatal to trigger Will.

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