Follow-up: Resolve the unchanged MQTT WebSocket authentication-completion race tracked by b96515b2...

Non-blocking follow-up raised by the Dispatch reviewer while reviewing HarperFast/harper#2419.

## Proposed work

Resolve the unchanged MQTT WebSocket authentication-completion race tracked by b96515b2ad2d91bd; it is non-blocking for this delta because it predates the pull request.

## Why this did not block the pull request

the reviewer reported this as adjacent work rather than a blocker

## Evidence from the review

Resolve the unchanged MQTT WebSocket authentication-completion race tracked by b96515b2ad2d91bd; it is non-blocking for this delta because it predates the pull request.

## Where

- Repository-level; the reviewer did not tie this to a single path.
- Domain: unclassified

## Origin

- Pull request: https://github.com/HarperFast/harper/pull/2419
- Reviewed head: ed68ac7ab00a9544d2f97dcfff247945f4f595c8
- Review round: 3 (discovery)
- Fingerprint: `fup-3515b1533ebe6138`

The issue names no file or test. Start by inspecting commit b96515b2ad2d91bd and the MQTT WebSocket authentication flow discussed in pull request #2419, then trace the authentication-completion race. Done means the race is resolved and its completion behavior is covered by regression checks.

Written by the indexing model from the issue text.