Added tests for utility functions in the pkg/util

[anujagrawal699]

Description:
This PR introduces comprehensive unit tests for utility functions in the pkg/util package. These additions aim to increase code reliability, maintainability, and test coverage for the context handling, proxy header round tripper, and policy activation logic.

Additions:

1. pkg/util/context_test.go
2. pkg/util/round_trippers_test.go
3. pkg/util/policy_test.go

Test Coverage:

1. pkg/util/context.go : 0% to 100%
2. pkg/util/round_trippers.go : 0% to 100%
3. pkg/util/policy.go : 0% to 100%

What type of PR is this?
/kind feature

Which issue(s) this PR fixes:
Fixes a part of #5470

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.98%. Comparing base (331145f) to head (d35b21e).
Report is 4 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5733      +/-   ##
==========================================
+ Coverage   40.90%   40.98%   +0.07%     
==========================================
  Files         650      650              
  Lines       55182    55182              
==========================================
+ Hits        22573    22615      +42     
+ Misses      31171    31130      -41     
+ Partials     1438     1437       -1     

Flag	Coverage Δ
unittests	40.98% <ø> (+0.07%)	⬆️

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[zhzhuang-zju]

/lgtm
cc @XiShanYongYe-Chang for another look

[XiShanYongYe-Chang]

/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: XiShanYongYe-Chang

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• pkg/util/OWNERS [XiShanYongYe-Chang]

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