Stop docker watcher in docker autodiscover

[belimawr]

Proposed commit message

The docker watcher in the docker autodiscover was not stopped and the Provider Stop method did not wait the provider goroutines to return.

That caused two problems:
 - improper shutdown
 - tests using the testing.T as logger output could panic.

This commit fixes it by calling stop in the watcher and waiting all started goroutines to finish.

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

Run the tests, use -count=2 or a larger value

cd libbeat/autodiscover/providers/docker
go test -count=1 -v -run=TestDockerStart ./... -tags=integration -count=2

Related issues

• Closes TestDockerStart - panic: Log in goroutine after TestDockerStart has completed #50545

## Use cases
## Screenshots
## Logs

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Enterprise

Run ID: d8343994-878f-49f9-86aa-58a8009700ae

📥 Commits

Reviewing files that changed from the base of the PR and between 2ddb445 and c848fab.

📒 Files selected for processing (1)

• libbeat/autodiscover/providers/docker/docker.go

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📝 Walkthrough

Walkthrough

This PR adds graceful shutdown coordination to the Docker autodiscover provider. The change introduces a sync.WaitGroup field to track the main event-loop goroutine's lifecycle. During Start(), the goroutine is launched via stopWg.Go() instead of a raw go func(). In Stop(), after signaling the goroutine to exit via the stop channel, the code now waits for the WaitGroup to complete before returning. This ensures the event-loop goroutine finishes its cleanup and stops logging before Stop() returns, addressing the test flake where goroutines log after test completion.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The pull request addresses all requirements from issue #50545: stops the docker watcher and ensures Provider.Stop() waits for goroutines via sync.WaitGroup to prevent logging after test completion.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the identified issue: sync.WaitGroup addition to Provider struct, goroutine coordination in Start/Stop methods, and no unrelated modifications.

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• 🛠️ Update Documentation: Create PR

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