fix(agents): use getattr for include_contents and consolidate include_sources validation

- contents.py: read agent.include_contents via getattr with 'default'
  fallback, fixing the mypy union-attr error (agent is typed BaseNode |
  None) and the AttributeError raised by spec'd Mock agents in tests
  (pydantic v2 field names are not class attributes, so they are absent
  from a MagicMock(spec=Agent)).
- llm_agent.py: fold the include_sources=[] check into the existing
  __model_validator_after instead of a separate field_validator. This
  avoids introducing a new untyped-decorator mypy error rather than
  suppressing it with type: ignore.

Author: bofenghuang

src/google/adk/agents/llm_agent.py

@@ -978,6 +978,10 @@ def __maybe_save_output_to_state(self, event: Event):
 
   @model_validator(mode='after')
   def __model_validator_after(self) -> LlmAgent:
+    if self.include_sources is not None and len(self.include_sources) == 0:
+      raise ValueError(
+          'include_sources=[] keeps nothing. Use None to disable filtering.'
+      )
     if self.include_contents == 'none' and self.include_sources is not None:
       warnings.warn(
           "include_contents='none' with include_sources may produce empty"
@@ -990,17 +994,6 @@ def __model_validator_after(self) -> LlmAgent:
       )
     return self
 
-  @field_validator('include_sources', mode='after')  # type: ignore[misc]
-  @classmethod
-  def _validate_include_sources(
-      cls, v: Optional[list[str]]
-  ) -> Optional[list[str]]:
-    if v is not None and len(v) == 0:
-      raise ValueError(
-          'include_sources=[] keeps nothing. Use None to disable filtering.'
-      )
-    return v
-
   @field_validator('generate_content_config', mode='after')
   @classmethod
   def validate_generate_content_config(

src/google/adk/flows/llm_flows/contents.py

@@ -69,7 +69,8 @@ async def run_async(
 
     is_single_turn = getattr(agent, 'mode', None) == 'single_turn'
     source_filter = getattr(agent, 'include_sources', None)
-    if agent.include_contents == 'default':
+    include_contents = getattr(agent, 'include_contents', 'default')
+    if include_contents == 'default':
       # Include full conversation history
       llm_request.contents = _get_contents(
           invocation_context.branch,
@@ -85,7 +86,7 @@ async def run_async(
       # 'current': anchor at last user message — all sibling agent outputs
       #            within this invocation are included.
       # 'none':    anchor at last turn boundary (user OR other-agent event).
-      stop_at_user_only = agent.include_contents == 'current'
+      stop_at_user_only = include_contents == 'current'
       llm_request.contents = _get_current_turn_contents(
           invocation_context.branch,
           invocation_context.session.events,