feat(cli): Add --auto_create_session flag to adk api_server CLI

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

src/google/adk/cli/adk_web_server.py

@@ -494,6 +494,7 @@ class AdkWebServer:
       logo_text: Optional[str] = None,
       logo_image_url: Optional[str] = None,
       url_prefix: Optional[str] = None,
+      auto_create_session: bool = False,
   ):
     self.agent_loader = agent_loader
     self.session_service = session_service
@@ -511,6 +512,7 @@ class AdkWebServer:
     self.current_app_name_ref: SharedValue[str] = SharedValue(value="")
     self.runner_dict = {}
     self.url_prefix = url_prefix
+    self.auto_create_session = auto_create_session
 
   async def get_runner_async(self, app_name: str) -> Runner:
     """Returns the cached runner for the given app."""
@@ -560,6 +562,7 @@ class AdkWebServer:
         session_service=self.session_service,
         memory_service=self.memory_service,
         credential_service=self.credential_service,
+        auto_create_session=self.auto_create_session,
     )
 
   def _instantiate_extra_plugins(self) -> list[BasePlugin]:

src/google/adk/cli/cli_tools_click.py

@@ -1419,6 +1419,14 @@ def cli_web(
 @fast_api_common_options()
 @adk_services_options(default_use_local_storage=True)
 @deprecated_adk_services_options()
+@click.option(
+    "--auto_create_session",
+    is_flag=True,
+    default=False,
+    help=(
+        "Automatically create a session if it doesn't exist when calling /run."
+    ),
+)
 def cli_api_server(
     agents_dir: str,
     eval_storage_uri: Optional[str] = None,
@@ -1439,6 +1447,7 @@ def cli_api_server(
     a2a: bool = False,
     reload_agents: bool = False,
     extra_plugins: Optional[list[str]] = None,
+    auto_create_session: bool = False,
 ):
   """Starts a FastAPI server for agents.
 
@@ -1471,6 +1480,7 @@ def cli_api_server(
           url_prefix=url_prefix,
           reload_agents=reload_agents,
           extra_plugins=extra_plugins,
+          auto_create_session=auto_create_session,
       ),
       host=host,
       port=port,

src/google/adk/cli/fast_api.py

@@ -91,6 +91,7 @@ def get_fast_api_app(
     extra_plugins: Optional[list[str]] = None,
     logo_text: Optional[str] = None,
     logo_image_url: Optional[str] = None,
+    auto_create_session: bool = False,
 ) -> FastAPI:
 
   # Set up eval managers.
@@ -153,6 +154,7 @@ def get_fast_api_app(
       logo_text=logo_text,
       logo_image_url=logo_image_url,
       url_prefix=url_prefix,
+      auto_create_session=auto_create_session,
   )
 
   # Callbacks & other optional args for when constructing the FastAPI instance