[FEATURE] agentgateway router reads its OTLP address from ate-otel-config #3666

The agentgateway router's OTLP address is hard-coded in its kustomization instead of read from the shared ate-otel-config ConfigMap. Every other component reads the endpoint from that ConfigMap, so a provider or operator who changes it still gets the GKE collector on the router.

Tracks "Make the OpenTelemetry endpoint provider-neutral", first step, in the Provider-Neutral Core workstream of the Extensible Install Roadmap.

Current state (0b635b80): manifests/ate-install/components/agentgateway/kustomization.yaml sets AGENTGATEWAY_OTLP_ADDRESS to the gke-managed-otel collector address.

Required changes

• Source AGENTGATEWAY_OTLP_ADDRESS from ate-otel-config (OTEL_EXPORTER_OTLP_ENDPOINT, converted if the router needs a different format), in the base component and in the kind agentgateway overlay.
• No change to the default value. Replacing the GKE default is Default OTLP endpoint points at an addon namespace that usually does not exist #1258, after the GKE parity release.

Dependencies: none. Foundation phase.

Breaking change: none. Rendered output is unchanged for default and kind installs.

Enforcement

• Render test: default and kind output unchanged.
• Render test: changing OTEL_EXPORTER_OTLP_ENDPOINT in ate-otel-config changes the router's address.

Parent Issue: #3161