Enhance `mock_clients.py` by Centralizing Mock Model Creation

Currently, the setup for mock Gemini models used in testing (e.g., in `test_gemini_client.py::TestGeminiClient::test_list_models_success`) is defined directly within the test file itself. This creates a tight coupling between the test logic and the mock data setup.

**Problem:**

1. **Coupling:** Test files become responsible for defining the structure and behavior of mock objects that represent external API responses.
2. **Duplication:** If other test modules need similar mock models, the setup logic might be duplicated, leading to inconsistencies and maintenance overhead.
3. **Organization:** The `mock_clients.py` helper is intended to centralize mock setup, but the model mocking is currently outside its scope.

**Proposed Solution:**

Refactor the `mock_clients.py` module to include functionality for generating and configuring mock Gemini models.

1. **Add Mock Model Data:** Define standard mock model structures (like those used in `test_list_models_success`) within `mock_clients.py`.
2. **Create Helper Function:** Introduce a function (e.g., `get_mock_gemini_models()`) in `mock_clients.py` that returns a pre-configured list of mock `genai.Model` objects.
3. **Update Tests:** Modify `test_gemini_client.py` (and potentially other future tests) to use this new helper function instead of defining mock models inline. The `setup_gemini_mocks` function could potentially be extended to configure the `mock_genai.list_models` return value directly.

**Benefits:**

* **Decoupling:** Test files focus solely on testing client logic, relying on the helper for mock data.
* **Reusability:** Mock model definitions are centralized and easily reusable across different test suites.
* **Maintainability:** Changes to the mock model structure only need to be made in one place (`mock_clients.py`).
* **Consistency:** Ensures all tests use consistently structured mock models.

**Acceptance Criteria:**

* Mock model definitions are removed from `test_gemini_client.py`.
* `mock_clients.py` contains the logic for creating and returning mock Gemini models.
* `test_gemini_client.py` utilizes the new helper functionality from `mock_clients.py` to get mock models for testing `list_models`.
* All relevant tests pass after the refactoring.