fix(tests): Append trailing newline to JSON test outputs

JSON test files generated by the agent test runner were missing trailing newlines, causing the pre-commit end-of-file-fixer hook to fail. Added an explicit newline write on serialization.

Change-Id: Idaffc4f2c23fa213f595ddef2188b0050aeb431d

Author: DeanChensj

contributing/samples/tools/parallel_functions/tests/test_all_tools.json

@@ -193,4 +193,4 @@
       }
     }
   ]
-}
+}

contributing/samples/tools/parallel_functions/tests/test_parallel_mixed.json

@@ -125,4 +125,4 @@
       }
     }
   ]
-}
+}

contributing/samples/tools/parallel_functions/tests/test_parallel_weather.json

@@ -126,4 +126,4 @@
       }
     }
   ]
-}
+}

contributing/samples/tools/pydantic_argument/tests/test_create_company.json

@@ -136,4 +136,4 @@
       }
     }
   ]
-}
+}

contributing/samples/tools/pydantic_argument/tests/test_create_user.json

@@ -101,4 +101,4 @@
       }
     }
   ]
-}
+}

contributing/samples/tools/pydantic_argument/tests/test_create_user_with_prefs.json

@@ -104,4 +104,4 @@
       }
     }
   ]
-}
+}

src/google/adk/cli/agent_test_runner.py

@@ -881,6 +881,7 @@ def get_next_fc_id():
       # Write back to file
       with open(test_file, "w") as f:
         json.dump(session_data, f, indent=2, sort_keys=True)
+        f.write("\n")
 
       print(f"Successfully rebuilt {test_file}")