Merge https://github.com/google/adk-python/pull/4117
**Overview**
This PR implements the feature request in #4108 to allow `thinking_config` to be set directly within `generate_content_config`, bringing the Python SDK in line with the Go implementation.
**Changes**
- **llm_agent.py**: Relaxed the validation logic in `validate_generate_content_config` to remove the `ValueError` for `thinking_config`.
- **Precedence Warning**: Added an override of `model_post_init` in `LlmAgent` to issue a `UserWarning` if both a `planner` and a manual `thinking_config` are provided.
- **built_in_planner.py**: Updated `apply_thinking_config` to log an `INFO` message when the planner overwrites an existing configuration on the `LlmRequest`.
**Testing**
Verified with a reproduction script covering:
1. Successful initialization of an agent with direct `thinking_config`.
2. Validation of `UserWarning` during initialization when conflicting configurations are present.
3. Confirmation of logger output when the planner performs an overwrite.
Closes: #4108
Tagging @invictus2010 for visibility.
Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4117 from Akshat8510:feat/allow-thinking-config-4108 5deeb893799379c681d6822dc4a1e42f86d3ed01
PiperOrigin-RevId: 856821447
