fix(firebaseai): Export UnknownPart (#17655)

* Export UnknownPart

* Remove unused import

Author: gspencergoog

packages/firebase_ai/firebase_ai/lib/firebase_ai.dart

@@ -43,7 +43,8 @@ export 'src/content.dart'
         FunctionCall,
         FunctionResponse,
         Part,
-        TextPart;
+        TextPart,
+        UnknownPart;
 export 'src/error.dart'
     show
         FirebaseAIException,

packages/firebase_ai/firebase_ai/test/utils/matchers.dart

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 import 'package:firebase_ai/firebase_ai.dart';
-import 'package:firebase_ai/src/content.dart';
 import 'package:http/http.dart' as http;
 import 'package:matcher/matcher.dart';