Add unit tests for server logic

[GautamBytes]

What this PR does / Why we need it
The pkg/webhook/server.go file contains the core logic for registering and managing webhook handlers. This PR introduces unit tests to validate this logic, ensure its correctness, and prevent future regressions.

Scope of Testing
These tests focus on the pure, stateless logic functions within the file:

• addHandlers() and addHandlersWithGate() are tested to ensure they correctly add handlers, normalize paths, and handle gates.
• filterActiveHandlers() is tested to verify that inactive handlers are correctly removed from the map based on their gate functions.

The integration functions (SetupWithManager, initialize), which are responsible for wiring together controller-runtime components and involve concurrency, have been intentionally excluded from this unit test suite. They are better suited for future integration tests.

[kruise-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.81%. Comparing base (00aae12) to head (797bdf8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   47.52%   47.81%   +0.28%     
==========================================
  Files          63       66       +3     
  Lines        9799     9864      +65     
==========================================
+ Hits         4657     4716      +59     
- Misses       4585     4589       +4     
- Partials      557      559       +2     

Flag	Coverage Δ
unittests	47.81% <100.00%> (+0.28%)	⬆️

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[GautamBytes]

@veophi can u help me with this linter failing workflow , i am running it locally but it still fails somehow , never happened for other prs!