Add unit tests for pod create/update handler

[GautamBytes]

Ⅰ. Describe what this PR does
This pull request introduces a unit test suite for the pod validating webhook handler located in pkg/webhook/pod/validating/pod_create_update_handler.go.

The primary goal is to increase test coverage and ensure the reliability of the webhook's core dispatch logic. The tests use a mock handler to verify that admission requests are correctly routed to either the WorkloadSpread or PodUnavailableBudget validation functions based on the operation type (CREATE, UPDATE, DELETE) and the state of their respective feature gates.

Ⅱ. Does this pull request fix one issue?
in support of #2074

Ⅲ. Describe how to verify it
You can verify this PR by running the tests for the specific package and ensuring they pass:
go test ./pkg/webhook/pod/validating -v
All newly added tests for pod_create_update_handler should pass successfully.

[kruise-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.02%. Comparing base (f69813c) to head (4354720).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2101      +/-   ##
==========================================
+ Coverage   43.99%   44.02%   +0.02%     
==========================================
  Files         316      316              
  Lines       31844    31844              
==========================================
+ Hits        14009    14018       +9     
+ Misses      16425    16418       -7     
+ Partials     1410     1408       -2     

Flag	Coverage Δ
unittests	44.02% <ø> (+0.02%)	⬆️

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