Add comprehensive unit tests for event handler

[GautamBytes]

What this PR does / Why we need it
The MutatingWebhookEventHandler in pkg/controller/deployment/deployment_event_handler.go contains critical logic for queueing Deployments but previously lacked unit test coverage. This made it difficult to refactor or protect against regressions.

This PR adds a comprehensive and robust test suite to validate the handler's functionality, ensuring stability and correctness.

Key Improvements & Scenarios Covered
Complete Handler Coverage: Tests have been added for all event handler methods (Create, Update, Delete, and Generic).

Content Verification: Tests now verify the content of the enqueued reconcile.Request (i.e., the correct deployment name and namespace), not just that the queue length has changed.

Bulk Enqueue Testing: A scenario with multiple matching deployments is tested to ensure the handler's loop logic is correct.

Error Handling: A new test, TestEnqueueErrorHandling, was added to simulate an API client error and confirm that the handler fails gracefully without adding items to the queue.

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.83%. Comparing base (00aae12) to head (8fcd407).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   47.52%   47.83%   +0.30%     
==========================================
  Files          63       63              
  Lines        9799     9799              
==========================================
+ Hits         4657     4687      +30     
+ Misses       4585     4553      -32     
- Partials      557      559       +2     

Flag	Coverage Δ
unittests	47.83% <ø> (+0.30%)	⬆️

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[GautamBytes]

@furykerry @veophi PTAL!