Client credentials still opens browser for authentication

**Describe the bug**
I am having the same problem as reported in this issue: https://github.com/google/adk-python/issues/1475. The steps to reproduce are exactly as reported there

**To Reproduce**
See the issue https://github.com/google/adk-python/issues/1475. The steps are the same

**Expected behavior**
Token exchange should happen server side. The browser must not open asking the user for authentication.

**Desktop (please complete the following information):**
- OS: macOS 26.0.1 (25A362)
- Python version(python -V): Python
- ADK version(pip show google-adk): 1.19.0

**Model Information:**
- Are you using LiteLLM: No
- Which model is being used: gemini-2.5-pro