Merge https://github.com/google/adk-python/pull/4288
**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):**
- Closes: #4274
**2. Or, if no issue exists, describe the change:**
N/A - Issue exists
### 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:**
- [X] I have added or updated unit tests for my change.
- [X] All unit tests pass locally.
$ pytest tests/unittests/cli/ -v
============================= test session starts ==============================
platform darwin -- Python 3.11.14, pytest-9.0.2, pluggy-1.6.0
collected 246 items
...
====================== 246 passed, 147 warnings in 21.38s ======================
**Manual End-to-End (E2E) Tests:**
1. Verify CLI flag is recognized:
$ adk api_server --help | grep auto_create_session --auto_create_session
Automatically create a session if it doesn't exist when calling /run.
2. Start server with flag enabled:
$ adk api_server --auto_create_session
3. Test /run endpoint without pre-creating session:
$ curl -X POST http://localhost:8000/run \
-H "Content-Type: application/json" \
-d '{"app_name": "my_agent", "user_id": "user1", "session_id": "new_session", "new_message": {"role": "user", "parts": [{"text": "Hello"}]}}'
Expected: Session auto-created, request succeeds (no 404 error).
### 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.
### Additional context
This PR exposes the existing Runner.auto_create_session functionality (added in commit 8e69a58 / ADK v1.23.0) through the adk api_server CLI command.
Files changed (3 files, ~15 lines):
src/google/adk/cli/cli_tools_click.py - Add --auto_create_session CLI option
src/google/adk/cli/fast_api.py - Pass parameter through get_fast_api_app()
src/google/adk/cli/adk_web_server.py - Store and use in _create_runner()
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4288 from ekimcodes:main 3c8d299a88b21789431dbda488befba7ee3ace81
PiperOrigin-RevId: 868361303
