Fix deletion guard for Gateway TargetGroupConfigurations still referenced by TCPRoutes

[immanuwell]

Issue

None

Description

Gateway-targeted TargetGroupConfigurations are a documented path for gateway chaining, but delete reconciliation only guarded Service targets.

So if a TCPRoute still points at a Gateway backend, deleting the TGC can drop the finalizer and remove the config while it is still referenced. Tiny edge case, but its real.

This patch adds the missing in-use check for targetReference.kind: Gateway and covers it with tests.

Repro:

1. Create a TargetGroupConfiguration with targetReference.kind: Gateway.
2. Create a TCPRoute that uses that Gateway as a backend.
3. Delete the TGC.
4. Before this patch, the controller can remove the finalizer anyway.
5. With this patch, deletion stays blocked until the TCPRoute reference is gone.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• 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 🌟

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: immanuwell
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[kubernetes-prow]

Hi @immanuwell. Thanks for your PR.

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[immanuwell]

@shraddhabang @shuqz please take a look