Validate MQTT 5 SUBSCRIBE identifiers

[YAJeff]

The MQTT 5 SUBSCRIBE decoder accepts Subscription Identifier zero and silently overwrites duplicate identifiers. Reject both as Protocol Errors, as required by MQTT 5.0 section 3.8.2.1.2: https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html.

Validation is local to each SUBSCRIBE packet. An absent identifier, valid boundary values, repeated User Properties, identifier reuse in separate SUBSCRIBE packets, and repeated identifiers in server-to-client PUBLISH remain accepted. No public API or MQTT 3 decoder changes.

Reproduction on master 18731d9: following a successful MQTT 5 CONNECT, SUBSCRIBE bytes 82 09 00 01 02 0B 00 00 01 61 00 receive SUBACK despite identifier zero. The fixed server closes the connection. Four synthetic negative regression cases fail on unmodified master; six positive cases pass.

Validation:

• 90 formatter tests pass on each of net8.0 and net10.0, including MQTT 3 serialization.
• Full net10.0 Release suite: 496 passed. The same-decoder positive test was then strengthened and all 90 formatter tests rerun successfully on both frameworks.
• git diff --check and decoder whitespace verification pass.

Release solution build also passed with three warnings in unchanged benchmark code.