follow up listenerset policy attachment

[kkk777-7]

What this PR does / why we need it:

Follow up for supporting ListenerSet policy attachment.

• perf: run translation for ListenerSet only when ListenerSet exist
  • similar to feat: envoy extension policy listenerset attach #9425 (in envoy extension policy)
• use common utils and remove unused functions

Which issue(s) this PR fixes:

Fixes #9607

---

PR Checklist

• Authorship & ownership: Coding agents / AI assistants are welcome, but I have reviewed every change, understand how and why it works, can explain and maintain it, and take full responsibility for this PR. I have not submitted generated output I do not understand.
• DCO: All commits are signed off (git commit -s). See DCO: Sign your work.
• API agreed first: If this PR contains API changes (changes under /api), the API was discussed and agreed before the implementation. The API change can be in a separate PR, or in the same PR, but the API must be agreed before implementation. N/A if this PR does not contain API changes.
• Required checks pass: make generate gen-check, make lint, and the unit-test/coverage build pass. (Flaky e2e failures are not considered breakages, but gen-check, lint, and coverage MUST pass.)
• Tests added/updated: New/changed code is covered by appropriate tests. N/A if this PR does not contain code changes.
• Docs: User-facing changes update the docs, either in this PR or a follow-up PR. N/A if this PR does not contain user-facing changes.
• Release notes: For any non-trivial change, added a release-note fragment under release-notes/current/<section>/<pr-number>-<slug>.md (see release-notes/current/README.md for sections and naming). N/A if this PR does not contain non-trivial changes.
• Generated files committed: Ran make gen-check and committed the result if API/helm charts/modules changed.
• Scope & compatibility: The PR is reasonably scoped (no unrelated changes) and preserves backward compatibility, or any breaking change is called out above and documented in release-notes/current/breaking_changes/.
• Codex review: Requested a Codex review and addressed all of its comments.
• Copilot review: Requested a Copilot review and addressed all of its comments.

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[codecov]

Codecov Report

❌ Patch coverage is 75.75758% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.04%. Comparing base (b710dd4) to head (0b57de8).

Files with missing lines	Patch %	Lines
internal/gatewayapi/backendtrafficpolicy.go	77.14%	6 Missing and 2 partials ⚠️
internal/gatewayapi/securitypolicy.go	74.19%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9621   +/-   ##
=======================================
  Coverage   76.04%   76.04%           
=======================================
  Files         259      259           
  Lines       43275    43258   -17     
=======================================
- Hits        32907    32896   -11     
+ Misses       8178     8173    -5     
+ Partials     2190     2189    -1     

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[kkk777-7]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 6e8cf34353

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[jukie]

LGTM, thanks!

[jukie]

/retest