fix: treat resource metadata JSON parse failure as soft error (#810)

In fetch_resource_metadata_from_url, a JSON parse failure on the
response body caused a fatal AuthError::MetadataError, preventing
discover_metadata() from falling through to direct
.well-known/oauth-authorization-server discovery (Strategy B).

MCP servers that return HTTP 200 with non-JSON content (e.g. HTML)
at their base URL caused the OAuth flow to abort entirely, even
when the server had a valid .well-known/oauth-authorization-server
endpoint.

Return Ok(None) on parse failure, consistent with how HTTP errors
are already handled in the same function.

Author: jh-block

crates/rmcp/src/transport/auth.rs

@@ -1609,12 +1609,13 @@ impl AuthorizationManager {
             return Ok(None);
         }
 
-        let metadata = response
-            .json::<ResourceServerMetadata>()
-            .await
-            .map_err(|e| {
-                AuthError::MetadataError(format!("Failed to parse resource metadata: {}", e))
-            })?;
+        let metadata = match response.json::<ResourceServerMetadata>().await {
+            Ok(metadata) => metadata,
+            Err(e) => {
+                debug!("failed to parse resource metadata as JSON: {}", e);
+                return Ok(None);
+            }
+        };
         Ok(Some(metadata))
     }