Fix missing changes in PR#709

[u-kai]

What type of PR is this?
Fixes a missing change from PR #709

Which issue does this PR fix:
Fixes a missing change from PR #709
What does this PR do / Why do we need it:
In PR #709, the logic was updated to traverse all Parents in XXXRoute. However, some parts of the implementation were still only checking the first element (ParentRefs[0]), which was an unintended omission.
This PR fixes the logic to correctly traverse all Parents and ensures consistency with the intended behavior.
Additionally, some refactoring was performed to improve maintainability.

If an issue # is not available please add repro steps and logs from aws-gateway-controller showing the issue:

Testing done on this change:

Verified that existing unit tests cover this change and all tests pass
Automation added to e2e:

None
Will this PR introduce any new dependencies?:

None
Will this break upgrades or downgrades. Has updating a running cluster been tested?:
None
Does this PR introduce any user-facing change?:

Do all end-to-end tests successfully pass when running make e2e-test?:

SECONDARY_ACCOUNT_TEST_ROLE_ARN=arn:aws:iam:xxxxxxxxxxxx:role/TestRole REGION=ap-northeast-1  make e2e-test
...
------------------------------
[SynchronizedAfterSuite] 
/Users/kai/aws-application-networking-k8s/test/suites/integration/suite_test.go:72
{"level":"info","ts":"2025-02-16T15:48:07.053+0900","caller":"test/framework.go:266","msg":"Deleting objects: *v1.Gateway/test-gateway, *v1.Pod/grpc-runner"}
{"level":"info","ts":"2025-02-16T15:48:07.090+0900","caller":"test/framework.go:285","msg":"Waiting for NotFound, objects: *v1.Gateway/test-gateway, *v1.Pod/grpc-runner"}
[SynchronizedAfterSuite] PASSED [31.100 seconds]
------------------------------

Ran 69 of 69 Specs in 2756.680 seconds
SUCCESS! -- 69 Passed | 0 Failed | 0 Pending | 0 Skipped
--- PASS: TestIntegration (2756.68s)
PASS
ok      github.com/aws/aws-application-networking-k8s/test/suites/integration   2757.433s

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rlymbur]

I've created #717 to address these failing validations.

[rlymbur]

Very minor comments, otherwise thank you for simplifying this!

[u-kai]

@rlymbur
Thanks for the review!
I have revised it according to your review.

[rlymbur]

LGTM 🚀