FallbackManager WaitForStop calls Stop

[belimawr]

Proposed commit message

Update the FallbackManager to call Stop in WaitForStop to be consistent with the method goal and all other implementations (real or mocks).

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

## Disruptive User Impact
## How to test this PR locally

Related issues

• Follow up from Split BeatV2Manager Start into two methods, so Beats can reply to check-in in parallel to its initialisation #49796

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Reviewing files that changed from the base of the PR and between 5b32798 and 0c42046.

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• libbeat/management/management.go

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📝 Walkthrough

Walkthrough

The WaitForStop() method in FallbackManager was updated to execute shutdown logic before returning. Previously, the method returned true immediately without triggering any stop operations. The modification adds a call to n.Stop() prior to the return statement, ensuring shutdown is actively initiated. The method signature and behavior of other FallbackManager methods remain unchanged. The modification affects 9 lines added and 6 lines removed.

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