fix: check Content-Type before parsing JSON in fetchExternal (#40958)

When auth is not configured, /.auth/me returns HTML instead of JSON,
causing a SyntaxError in the browser console. Added Content-Type
validation to throw a descriptive error that the catch block in
appSlice handles gracefully, falling back to anonymous user.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Pavan-Microsoft

src/App/src/utils/httpClient.ts

@@ -68,6 +68,10 @@ class HttpClient {
     if (!response.ok) {
       throw new Error(`${config.method || 'GET'} ${url} failed: ${response.statusText}`);
     }
+    const contentType = response.headers.get('content-type') || '';
+    if (!contentType.includes('application/json')) {
+      throw new Error(`${config.method || 'GET'} ${url} returned non-JSON response (${contentType})`);
+    }
     return response.json();
   }
 }