TestE2E/PreserveCase flaky

```console
=== RUN TestE2E/PreserveCase
conformance.go:72: 2025-05-08T11:25:15.847728215Z: Applying testdata/preserve-case.yaml
apply.go:275: 2025-05-08T11:25:15.861242602Z: Creating gateway-preserve-case-backend Namespace
apply.go:275: 2025-05-08T11:25:15.871427685Z: Creating allow-preserve-case ReferenceGrant
apply.go:275: 2025-05-08T11:25:15.882614393Z: Creating preserve-case ClientTrafficPolicy
apply.go:275: 2025-05-08T11:25:15.888289118Z: Creating preserve-case HTTPRoute
apply.go:275: 2025-05-08T11:25:15.903823951Z: Creating preserve-case Deployment
apply.go:275: 2025-05-08T11:25:15.921650653Z: Creating fasthttp-backend Service
=== RUN TestE2E/PreserveCase/should_preserve_header_cases_in_both_directions
helpers.go:611: 2025-05-08T11:25:15.979804281Z: Route gateway-conformance-infra/preserve-case expected 1 Parents got 0
helpers.go:611: 2025-05-08T11:25:16.960642579Z: Conditions matched expectations
helpers.go:611: 2025-05-08T11:25:16.960695818Z: Route gateway-conformance-infra/preserve-case Parents matched expectations
utils.go:58: 2025-05-08T11:25:16.96306644Z: waiting for gateway-preserve-case-backend/[map[app:preserve-case]] to be Running...
preservecase.go:129: 2025-05-08T11:25:18.974304804Z: Making GET request to http://[fc00:f853:ccd:e793::cb]/preserve?headers=ReSpOnSeHeAdEr
Sending Request:
< GET /preserve?headers=ReSpOnSeHeAdEr HTTP/1.1
< Host: [fc00:f853:ccd:e793::cb]
< User-Agent: Go-http-client/1.1
< SpEcIaL: Header
< X-Echo-Set-Header:
< Accept-Encoding: gzip
<
<

Received Response:
< HTTP/1.1 503 Service Unavailable
< Date: Thu, 08 May 2025 11:25:18 GMT
< Content-Length: 0
<
<

preservecase.go:132: failed to get expected response: unexpected error reading response: unexpected end of JSON input
preservecase.go:136: case was not preserved for test header: map[]
=== NAME TestE2E/PreserveCase
apply.go:283: 2025-05-08T11:25:18.977177117Z: Deleting fasthttp-backend Service
apply.go:283: 2025-05-08T11:25:19.028798334Z: Deleting preserve-case Deployment
apply.go:283: 2025-05-08T11:25:19.039109934Z: Deleting preserve-case HTTPRoute
apply.go:283: 2025-05-08T11:25:19.050509631Z: Deleting preserve-case ClientTrafficPolicy
apply.go:283: 2025-05-08T11:25:19.065401978Z: Deleting allow-preserve-case ReferenceGrant
apply.go:283: 2025-05-08T11:25:19.082246632Z: Deleting gateway-preserve-case-backend Namespace
```