feat(gateway): implement validation and status tracking for TargetGroupConfiguration

[Nithin0620]

PR Title:

feat(gateway): implement validation and status tracking for TargetGroupConfiguration

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Description

This PR addresses technical debt and improves the reliability of the TargetGroupConfiguration (TGC) API and controller. It completes several TODO items related to API validation and resource observability.

Key Changes:

1. API Validation (HealthCheckMatcher):

   • Added a +kubebuilder:validation:XValidation rule to the HealthCheckMatcher struct.
   • This enforces that exactly one of httpCode or grpcCode is specified, providing immediate "fail-fast" validation at the Kubernetes API server level.

2. Controller Logic (ObservedGeneration):

   • Implemented the ObservedGeneration field in TargetGroupConfigurationStatus.
   • Updated the TGC reconciler to correctly populate this field during the reconciliation loop. This allows users and automation tools to verify that the controller has successfully processed the latest version of the resource.

3. Automated Testing:

   • Added a new unit test suite: targetgroup_configuration_controller_test.go.
   • Verified that the controller correctly identifies generation changes and updates the status subresource as expected.

4. Refactoring:

   • Cleaned up associated TODO comments and standardized status field documentation to match existing patterns in the codebase.

Checklist

• Added tests that cover your change
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested (Verified via go test)
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟 (Converted TODO comments into functional validation and status logic!)

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Nithin0620
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[Nithin0620]

@shraddhabang @wweiwei-li
This PR has been ready for review for a while now. Would appreciate a review when possible. Thanks!