fix(workflow): Preserve explicit single-turn contents

Merge #6047

**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: #6046

**Problem:**

`single_turn` `LlmAgent` workflow nodes currently force `include_contents` to `none` at runtime, even when the user explicitly configures `include_contents="default"`. That silently changes the agent configuration and prevents one-shot workflow nodes from intentionally receiving relevant session or workflow history.

**Solution:**

Only apply the workflow `single_turn` default of `include_contents="none"` when `include_contents` was not explicitly set by the user. This preserves the existing implicit default behavior while respecting explicit `include_contents="default"` and `include_contents="none"` values.

Regression coverage was added for implicit and explicit `include_contents` cases.

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

Targeted workflow tests pass locally:

```shell
uv run pytest tests/unittests/workflow/test_llm_agent_as_node.py -q
```

Result:

```text
28 passed, 8 skipped, 4 xfailed
```

Python 3.10 tox unit suite passes locally:

```shell
uv tool run --from tox --with tox-uv tox -e py310
```

Result:

```text
7044 passed, 21 skipped, 31 xfailed, 9 xpassed
```

Pre-commit hooks pass for changed files:

```shell
uv run pre-commit run --files src/google/adk/workflow/_llm_agent_wrapper.py tests/unittests/workflow/test_llm_agent_as_node.py
```

**Manual End-to-End (E2E) Tests:**

Validated manually with a local workflow application that uses a `single_turn` workflow `LlmAgent` configured with `include_contents="default"`. The run confirmed that the explicit contents mode is preserved instead of being overwritten to `none`.

No ADK Web or UI behavior is changed.

### 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

No documentation update was made because this change does not add a new user-facing API or guide behavior; it preserves explicit configuration that was already supported by `LlmAgent.include_contents`.

No dependent changes are required.

Co-authored-by: Bo Yang <ybo@google.com>
COPYBARA_INTEGRATE_REVIEW=#6047 from spectaclehong:fix/single-turn-include-contents a98ad26
PiperOrigin-RevId: 931343211

Author: 2 people

.agents/skills/adk-unit-guide/SKILL.md

@@ -50,6 +50,7 @@ Use the following structure and instructions to create the guide for the code un
 - Present a single, minimum implementation of the code unit to demonstrate its use.
 - Show enough of the containing classes to make it clear where the code could be used.
 - Use unit test code as a starting point for the code example, if available.
+- When writing a sample agent, do not set the `model` attribute.
 
 ## How it works
 

src/google/adk/workflow/_llm_agent_wrapper.py

@@ -296,7 +296,8 @@ async def run_llm_agent_as_node(
         f" but agent '{agent.name}' has mode='{agent.mode}'."
     )
 
-  if agent.mode == 'single_turn':
+  include_contents_explicit = 'include_contents' in agent.model_fields_set
+  if agent.mode == 'single_turn' and not include_contents_explicit:
     agent.include_contents = 'none'
 
   agent_ctx = prepare_llm_agent_context(agent, ctx)

tests/unittests/workflow/test_llm_agent_as_node.py

@@ -246,6 +246,71 @@ def test_default_mode_auto_set_to_single_turn(self):
 
     assert node.mode == 'single_turn'
 
+  @pytest.mark.parametrize(
+      ('agent_kwargs', 'expected_include_contents'),
+      [
+          ({}, 'none'),
+          ({'mode': 'single_turn'}, 'none'),
+          (
+              {'mode': 'single_turn', 'include_contents': 'default'},
+              'default',
+          ),
+          ({'mode': 'single_turn', 'include_contents': 'none'}, 'none'),
+      ],
+  )
+  @pytest.mark.asyncio
+  async def test_single_turn_defaults_include_contents_only_when_unset(
+      self,
+      monkeypatch: pytest.MonkeyPatch,
+      agent_kwargs: dict[str, Any],
+      expected_include_contents: str,
+  ):
+    """Single-turn workflow nodes preserve explicit content inclusion."""
+    from unittest.mock import MagicMock
+
+    from google.adk.workflow import _llm_agent_wrapper
+
+    agent = LlmAgent(
+        name='test_agent',
+        model='gemini-2.5-flash',
+        instruction='Test.',
+        **agent_kwargs,
+    )
+    wrapper = build_node(agent)
+    seen_include_contents = []
+
+    async def mock_run_async(*args, **kwargs):
+      seen_include_contents.append(wrapper.include_contents)
+      yield Event(
+          invocation_id='inv',
+          author=wrapper.name,
+          content=types.Content(parts=[types.Part(text='ok')]),
+      )
+
+    object.__setattr__(wrapper, 'run_async', mock_run_async)
+    monkeypatch.setattr(
+        _llm_agent_wrapper,
+        'prepare_llm_agent_context',
+        lambda agent, ctx: ctx,
+    )
+    monkeypatch.setattr(
+        _llm_agent_wrapper,
+        'prepare_llm_agent_input',
+        lambda agent, ctx, node_input: None,
+    )
+    ctx = MagicMock(spec=Context)
+    ic = MagicMock()
+    ctx.get_invocation_context.return_value = ic
+    ic.model_copy.return_value = ic
+
+    events = [
+        event async for event in wrapper._run_impl(ctx=ctx, node_input='hi')
+    ]
+
+    assert seen_include_contents == [expected_include_contents]
+    assert wrapper.include_contents == expected_include_contents
+    assert events[0].content.parts[0].text == 'ok'
+
   def test_name_override(self):
     """build_node respects explicit name override."""
     node = build_node(_make_agent(mode='task'), name='override')