Add Agent Registry Service and Binding to GA

[belovezhalin]

Promote Agent Registry Service and Binding resources from nightly (alpha) targets to General Availability

Changes

• Promoted Version: Added ga version to mmv1/products/agentregistry/product.yaml with the v1 endpoint.
• Resources promoted: Updated Service.yaml and Binding.yaml to specify min_version: ga and adjusted example configuration paths.
• Go Code & Docs Ported: Transferred the custom datasource files and markdown documentation from private overrides into the public structure.
• Test templates: Added basic example templates under mmv1/templates/terraform/examples/.

Verified local generation checks out cleanly.

Internal cleanup: cl/934347420

`google_agent_registry_service`: Added the `google_agent_registry_service` resource to GA.

`google_agent_registry_binding`: Added the `google_agent_registry_binding` resource to GA.

`google_agent_registry_endpoint`: Added the `google_agent_registry_endpoint` data source.

`google_agent_registry_endpoint`: Added the `google_agent_registry_endpoint` data source.

`google_agent_registry_mcp_server`: Added the `google_agent_registry_mcp_server` data source.

[google-cla]

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