Fix MQTT message error handling and improve connection responses

[coderzc]

Motivation

Currently, when there is an exception in checking the connect message, we directly close the channel. However, the client might keep retrying indefinitely. It's best for us to return a connection error to the client.

Modifications

For CONNECT message errors, send a CONNACK error response then close channel.
For other message errors, send a DISCONNECT error response if supported then close channel.

Verifying this change

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This change is a trivial rework / code cleanup without any test coverage.

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

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[github-actions]

@coderzc:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)