chore: add missing Apache 2.0 license headers

[milenvk]

Please ensure you have read the contribution guide 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.
• 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

• I have read the CONTRIBUTING.md document.
• 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.
• 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

[DeanChensj]

Hi @milenvk , thanks for your contribution, we explicitly excluded those files from the header check in https://github.com/google/adk-python/blob/main/.github/header-checker-lint.yml

Is the policy enforced that even sample config file and js assets need a header?