Merge https://github.com/google/adk-python/pull/4705
### Link to Issue or Description of Change
- Closes: #4666
**Problem:**
Gemini-2.0-flash model is still used in integration tests, but disabled for new users
**Solution:**
Replace all occurrences of Gemini-2.0-flash by Gemini-2.5-flash, in integration tests and docstrings.
Because gemini-2.0 is still enabled for some users, this PR doesn't clean the model from unit tests.
### Testing Plan
Run already existing unittests and integration tests
### Checklist
- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4705 from morganroux:fix/issues/4666 3c390a2ed3abb7b2fc7bd99cfb801e8a2b1f5343
PiperOrigin-RevId: 900973552
Change-Id: I1d2dd4c749522b529537d5863e4a56865927da95
Morgan Roux
17f3a8a929