feat: Add test suite for examples/planning and ensure v19 API usage

This commit introduces a test suite for the Python scripts located in the
`examples/planning` directory. It also ensures that the Google Ads API
client is configured to use version v19.

Key changes include:

1.  **API Version Update:**
    *   Modified `google-ads.yaml` to set the default API version to `v19`.

2.  **Test Suite Creation:**
    *   Created a new directory `examples/planning/tests` to house the tests.
    *   Added an empty `__init__.py` file to `examples/planning/tests` to make it a Python package.
    *   Developed individual test files for each script in `examples/planning`:
        *   `test_forecast_reach.py`
        *   `test_generate_forecast_metrics.py`
        *   `test_generate_historical_metrics.py`
        *   `test_generate_keyword_ideas.py`
        *   `test_get_ad_group_criterion_cpc_bid_simulations.py`
    *   The tests utilize `unittest.mock` to simulate API calls and verify the core logic of each example script.

3.  **Testing and Refinement:**
    *   Executed the test suite and resolved initial issues, including:
        *   Missing dependencies (`google-api-core`).
        *   Incorrect assertions for client loading when the client is injected.
        *   Adjustments to mock object behavior for enums and list attributes to ensure accurate test outcomes.
    *   All tests are confirmed to pass.

The example scripts themselves in `examples/planning` already contained
`GoogleAdsClient.load_from_storage(version="v19")`, so no further code
modifications were needed in these files for v19 client initialization.

Author: google-labs-jules[bot]