Merge https://github.com/google/adk-python/pull/6155
**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**
### Link to Issue or Description of Change
**1. Link to an existing issue (if applicable):**
N/A
**2. Or, if no issue exists, describe the change:**
**Problem:**
The repository is missing Apache 2.0 license headers across 155 files. This is required by Google OSPO.
**Solution:**
Prepended the standard Apache 2.0 open-source license header to all flagged files. The copyright year was dynamically assigned based on each file's first commit year in git history.
### Testing Plan
_Please describe the tests that you ran to verify your changes. This is required for all PRs that are not small documentation or typo fixes._
**Unit Tests:**
- [ ] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.
_Please include a summary of passed `pytest` results._
N/A - This PR only adds comments (license headers) to the top of files and contains zero functional code changes. No new tests are needed, and existing tests should pass as-is.
**Manual End-to-End (E2E) Tests:**
N/A - No functional code was changed.
### 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.
- [ ] I have commented my code, particularly in hard-to-understand areas.
- [ ] 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.
- [ ] I have manually tested my changes end-to-end.
- [ ] Any dependent changes have been merged and published in downstream modules.
### Additional context
Co-authored-by: Shangjie Chen <deanchen@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/6155 from milenvk:fix-missing-licenses d2a264b9a812fd997daf975ec37996e6b4b7a042
PiperOrigin-RevId: 936165783
George Weale