refactor: simplify function definition for resolve_provider_and_model

[Iamsdt]

This pull request focuses on improving test reliability and code style in the codebase. The most significant changes ensure that environment variables do not cause nondeterministic behavior in tests, and unnecessary dependencies are removed for cleaner test files.

Test environment reliability:

• The test setup (setup_test_environment in tests/conftest.py) now explicitly unsets the GOOGLE_GENAI_USE_VERTEXAI environment variable to prevent accidental inheritance from developer environments. This ensures deterministic provider selection in tests and avoids unintended use of Vertex AI unless explicitly specified.

Test code cleanup:

• Removed the dotenv dependency and its usage from tests/normal_tests/test_custom_state_comprehensive.py, as environment variable loading is now handled centrally in the test setup.

Code style and formatting:

• Minor formatting change to the resolve_provider_and_model function signature in agentflow/core/llm/client_factory.py for consistency and readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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