Fenrir fixes

[embhorn]

• Fix f-3627: test BrokerBufCompare length-fold auth backstop
• Fix f-2359: preserve v5 SUBSCRIBE options through encode/decode
• Fix f-3394: scrub CONNECT credentials from broker rx_buf after auth
• Fix f-6656: scrub v5 CONNACK auth data from client rx_buf

[Copilot]

Pull request overview

Updates the broker CONNECT authentication test suite to better cover a constant-time credential comparison edge case and to align comments with the broker’s actual CONNACK return code for bad credentials.

Changes:

• Corrects auth-failure CONNACK code references in existing tests/comments to 0x04 (Bad user name or password).
• Adds a new mutation-guard test that fails if the length-fold backstop in BrokerBufCompare is removed (repeating-byte, different-length username case).
• Registers the new test in the broker auth test runner.

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