Add unit tests for PreviewFeature with bug fixes

[Copilot]

This PR adds comprehensive unit tests for the PreviewFeature type in toolkit/tools/imagecustomizerapi/previewfeaturetype.go to improve code coverage as requested in the issue.

Changes Made

Bug Fixes

• Fixed missing type declaration: Added missing PreviewFeature type to PreviewFeatureReinitializeVerity constant
• Fixed validation logic: Added PreviewFeatureReinitializeVerity to the IsValid() method's switch statement

Unit Tests Added

Created previewfeaturetype_test.go with 8 comprehensive tests:

• Individual validation tests for all 6 valid preview features:
  • PreviewFeatureUki
  • PreviewFeatureOutputArtifacts
  • PreviewFeatureInjectFiles
  • PreviewFeatureReinitializeVerity
  • PreviewFeaturePackageSnapshotTime
  • PreviewFeatureKdumpBootFiles
• Error case tests for invalid and empty values

Test Results

• ✅ All 8 new tests pass
• ✅ 100% coverage of PreviewFeature.IsValid() method
• ✅ No existing tests broken
• ✅ Overall package coverage: 82.9%

Code Quality

• Follows existing test patterns using stretchr/testify
• Minimal, surgical changes (only 3 lines modified in existing code)
• No deletion of working code
• Passes go vet and go fmt

The tests ensure that all valid preview features are properly validated and that invalid values are correctly rejected with appropriate error messages.

Fixes #279.

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[cwize1]

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