health check probes blocked indefinitely when EDS initialFetchTimeout is 0s

[yuehaii]

Title: health check probes blocked indefinitely when EDS initialFetchTimeout is 0s

Description:
After I set EDS initialFetchTimeout as 0s, all envoy gateway conformance and e2e test cases failed in waiting for health check activity to appear.

Repro steps:

1. set initialFetchTimeout: 0s
2. rerun all envoy gateway e2e test cases

Logs:

=== RUN TestE2E/BackendHealthCheckActiveHTTP/active_http
helpers.go:821: 2026-08-11T12:51:31.009924331Z: Conditions matched expectations
helpers.go:821: 2026-08-11T12:51:31.009977872Z: Route gateway-conformance-infra/http-with-health-check-active-http-pass Parents matched expectations
…
backend_health_check.go:112: 2026-08-11T12:52:40.144747982Z: cluster fail health check: new failure count: 2 (total: 2)
http.go:306: 2026-08-11T12:52:41.046201267Z: timeout while waiting after 60 attempts, 0/3 successes

=== RUN TestGatewayAPIConformance/BackendTLSPolicyConflictResolution/BackendTLSPolicies_targeting_the_same_Service_with_and_without_a_section_name/HTTP_request_sent_to_Service_using_the_BackendTLSPolicy_with_section_name_should_succeed
backendtlspolicy-conflict-resolution.go:130: 2026-08-11T12:52:47.817510589Z: Making GET request to host abc.example.com via http://172.18.0.200/backendtlspolicy-not-conflicted-with-section-name
http.go:325: 2026-08-11T12:52:47.828189571Z: Response expectation failed for request: {URL: {Scheme:http Opaque: User: Host:172.18.0.200 Path:/backendtlspolicy-not-conflicted-with-section-name Fragment: RawQuery: RawPath: RawFragment: ForceQuery:false OmitHost:false}, Host: abc.example.com, Protocol: HTTP, Method: GET, Headers: map[X-Echo-Set-Header:[]], UnfollowRedirect: false, ServerName: , ServerCertificate: , ClientCertificate: , ClientCertificateKey: } not ready yet: expected status code to be one of [200], got 400. CRes: &{400 -1 HTTP/1.1 map[Date:[Tue, 11 Aug 2026 12:52:47 GMT]] []} (after 1.732µs)

[tonya11en]

cc @botengyao