Use corev1.PullPolicy instead of string for EmbeddingServer ImagePullPolicy

[Sanskarzz]

Summary

Updates EmbeddingServerSpec.ImagePullPolicy to use corev1.PullPolicy instead of a plain string.

This keeps the existing kubebuilder enum validation and JSON schema behavior unchanged, while giving Go code compile-time safety and consistency with upstream Kubernetes container image pull policy types.

Fixes #4616

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Changes

• Change EmbeddingServerSpec.ImagePullPolicy from string to corev1.PullPolicy
• Update GetImagePullPolicy() to return corev1.PullPolicy
• Remove the controller-side cast when assigning the container image pull policy
• Update unit and integration tests to use corev1.PullAlways, corev1.PullNever, and corev1.PullIfNotPresent

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.97%. Comparing base (6733a54) to head (ae19880).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5240   +/-   ##
=======================================
  Coverage   67.97%   67.97%           
=======================================
  Files         612      612           
  Lines       62723    62723           
=======================================
+ Hits        42633    42636    +3     
+ Misses      16908    16899    -9     
- Partials     3182     3188    +6     

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