Update stacklok/toolhive-registry-server to v1.5.3

.github/upstream-projects.yaml

@@ -29,7 +29,7 @@
 projects:
   - id: toolhive-registry-server
     repo: stacklok/toolhive-registry-server
-    version: v1.5.2
+    version: v1.5.3
     docs_paths:
       - docs/toolhive/guides-registry
       - docs/toolhive/concepts/registry-criteria.mdx

docs/toolhive/guides-registry/configuration.mdx

@@ -258,20 +258,26 @@ registries:
 
 **Configuration options:**
 
-- `endpoint` (required): Base URL of the upstream MCP Registry API (without
-  path)
+- `endpoint` (required): Base URL of the upstream MCP Registry API, without the
+  `/v0.1/...` API paths
 - `timeout` (optional): Per-request HTTP timeout as a Go duration (for example,
   `30s`, `1m`). Must be greater than zero and no larger than `5m`. Defaults to
   `10s`. Raise this for public or occasionally slow upstreams whose
   time-to-first-byte exceeds the default. The same bound is enforced whether the
   source is loaded from the config file or created through the admin API.
 
-:::note
-
 The server automatically appends the appropriate API paths (`/v0.1/servers`,
-etc.) to the endpoint URL.
-
-:::
+etc.) to the endpoint URL. Before syncing, it checks the endpoint by requesting
+`/v0.1/servers?limit=1` and confirming the response contains a `servers` array.
+Any upstream that implements the MCP Registry v0.1 API passes this check,
+including another ToolHive Registry Server. To sync from another Registry
+Server, include the registry path in the endpoint, for example
+`https://registry.example.com/registry/default`.
+
+API sources don't send credentials, so the upstream must allow anonymous read
+access. If the check fails, the `syncStatus.message` field in the source's
+details (`GET /v1/sources/{name}`) shows the underlying error, such as an HTTP
+404 or 401 response.
 
 ### File source
 

docs/toolhive/guides-registry/deploy-operator.mdx

@@ -253,7 +253,12 @@ spec:
 ```
 
 The controller automatically appends the appropriate API paths to the endpoint
-URL.
+URL. The endpoint can be any service that implements the MCP Registry v0.1 API
+and allows anonymous reads. To sync from another ToolHive Registry Server,
+include its registry path in the endpoint, for example
+`https://registry.example.com/registry/default`. See
+[API endpoint source](./configuration.mdx#api-endpoint-source) for how the
+server validates the endpoint.
 
 ### Kubernetes discovery source