[tests] add tests in inplace_update_test.go

[patilpratik1905]

Ⅰ. Describe what this PR does

This PR adds unit tests to improve coverage of the inplace_update_test used for generating and applying Pod patch updates.
Added the below functions

• TestDefaultGeneratePatchBodyFunc
  Validates patch generation by comparing Sandbox and Pod container images, ensuring patches are created only when required and correctly handle partial updates, sidecars, and empty container specs.

• TestNewInPlaceUpdateControl
  Verifies proper initialization of the in-place update controller with different patch function configurations, ensuring the client and patch function are set correctly.

• TestInPlaceUpdateControl_Update_WithCustomPatchFunc
  Ensures the update operation safely returns a no-op when a custom patch function produces an empty patch, without errors or unnecessary API calls.

Ⅱ. Does this pull request fix one issue?

No

Ⅲ. Describe how to verify it

Ran make test to check coverage of file

• Before :

• After :

Ⅳ. Special notes for reviews

• Changes are kept minimal and scoped to inplace_update_test.go for easier review.
• Please let me know if the updated test expectations should be adjusted further to match intended behavior.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.93%. Comparing base (10d5f54) to head (7c118bc).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   48.71%   51.93%   +3.22%     
==========================================
  Files          85       87       +2     
  Lines        4701     4785      +84     
==========================================
+ Hits         2290     2485     +195     
+ Misses       2208     2089     -119     
- Partials      203      211       +8     

Flag	Coverage Δ
unittests	51.93% <ø> (+3.22%)	⬆️

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