fix: resolve mypy and lint errors for jinja2 templating

- Annotate the inner _artifact helper with a -> str return type to satisfy
  mypy --strict (no-untyped-def).
- Assign render_async() result to a str-typed variable before returning so
  mypy does not flag returning Any from a str-typed function (no-any-return).
- Collapse the Jinja2 conditional test template onto a single line to match
  pyink formatting.

Author: vaibhav-patel

src/google/adk/utils/instructions_utils.py

@@ -216,7 +216,7 @@ async def _render_with_jinja2(
   invocation_context = readonly_context._invocation_context
   session_state = invocation_context.session.state
 
-  async def _artifact(name: str):
+  async def _artifact(name: str) -> str:
     if invocation_context.artifact_service is None:
       raise ValueError('Artifact service is not initialized.')
     artifact = await invocation_context.artifact_service.load_artifact(
@@ -229,10 +229,11 @@ async def _artifact(name: str):
 
   env = SandboxedEnvironment(enable_async=True)
   jinja_template = env.from_string(template)
-  return await jinja_template.render_async(
+  rendered: str = await jinja_template.render_async(
       state=session_state,
       artifact=_artifact,
   )
+  return rendered
 
 
 def _is_valid_state_name(var_name):

tests/unittests/utils/test_instructions_utils.py

@@ -285,8 +285,7 @@ async def test_inject_session_state_jinja2_basic_substitution():
 @pytest.mark.asyncio
 async def test_inject_session_state_jinja2_conditional():
   instruction_template = (
-      "{% if state['is_premium'] %}Premium user.{% else %}Free user."
-      "{% endif %}"
+      "{% if state['is_premium'] %}Premium user.{% else %}Free user.{% endif %}"
   )
   premium_context = await _create_test_readonly_context(
       state={"is_premium": True}