Fix: Add and debug test suite for examples/travel

This commit completes the creation and debugging of a test suite for Python scripts in the examples/travel/ directory.

Summary of steps and changes:

1.  **Initial Test Creation**:
    *   I created the `examples/travel/tests/` directory and `__init__.py`.
    *   I generated initial test files for all 5 scripts:
        *   `test_add_hotel_ad.py`
        *   `test_add_hotel_ad_group_bid_modifiers.py`
        *   `test_add_hotel_listing_group_tree.py`
        *   `test_add_performance_max_for_travel_goals_campaign.py`
        *   `test_add_things_to_do_ad.py`

2.  **Debugging and Refinement**:
    *   **Common Test Structure**: I refactored the tests to remove `test_script_runner` methods, which unreliably tested `if __name__ == "__main__":` blocks. Instead, I focused `test_main` methods on the core script logic, directly passing a mocked `GoogleAdsClient` instance.
    *   **Script Correction**: I identified and fixed f-string errors in print statements within `examples/travel/add_hotel_ad.py`.
    *   **Enum Mocking**: I resolved an `AttributeError` in `test_add_performance_max_for_travel_goals_campaign.py` by ensuring mocked enum objects correctly provided a `.name` string attribute.
    *   **Mocking Complexities (`test_add_hotel_listing_group_tree.py`)**:
        *   I encountered significant difficulties mocking protobuf `Message.CopyFrom()` calls and reliably asserting direct attribute assignments on nested `MagicMock` objects.
        *   **Resolution**: I simplified assertions for this test to focus on reliably verifiable aspects: service call invocation, correct `customer_id`, expected number of operations, calls to `client.get_type` for key messages, calls to `client.copy_from` (the client utility, not the protobuf method), and print statements. This acknowledges the mocking limitations while still providing value.
    *   **Dynamic Type Mocking**: For `test_add_things_to_do_ad.py` and `test_add_performance_max_for_travel_goals_campaign.py`, I implemented a dynamic side effect for `client.get_type` to ensure that mocked types had necessary child attributes (as `MagicMock` instances) pre-configured. This prevented `AttributeError`s when the script accessed nested attributes on these mocked types.

3.  **Final Verification**:
    *   I confirmed that all individual test files passed.
    *   I ran `python -m unittest discover ./examples/travel/tests`, and all 5 tests passed successfully.

All planned work for this issue is now complete and included in this commit.

Author: google-labs-jules[bot]