OIDC SecurityPolicy: failed OIDC discovery returns HTTP 500 on all affected routes

*Description*:

When a SecurityPolicy configures OIDC by specifying only the issuer (relying on automatic discovery of the remaining endpoints via the provider's `/.well-known/openid-configuration` document), a failure of that discovery step causes every HTTPRoute protected by that OIDC policy to return HTTP 500 directly.

On large, shared clusters where many routes reference the same issuer, a single discovery failure breaks hundreds of routes simultaneously.

*Repro steps*:

* Create one or more OIDC SecurityPolicy resources that specify only `provider.issuer` (no explicit authorizationEndpoint / tokenEndpoint), targeting HTTPRoutes.
* Cause OIDC discovery to fail — e.g. the IdP's `/.well-known/openid-configuration` is temporarily unreachable, returns a non-2xx, times out, or the issuer is briefly misconfigured.
* Send requests to any HTTPRoute covered by those policies.

*Environment*:

* Envoy Gateway version: 1.8.1
* Kubernetes version: v1.35.5