docs(workflow): make the single-turn node example runnable

[winter-street]

Link to Issue or Description of Change

2. Or, if no issue exists, describe the change:

Problem:

The single-turn example in this guide does not run. There are three separate issues in the same snippet:

1. from google.adk.workflow import Workflow, build_node — google/adk/workflow/__init__.py does not export build_node (it is not in __all__ or _LAZY_MEMBERS, and there is no workflow/build_node.py), so the import raises ImportError.
2. writer_node = build_node(writer_agent) — unnecessary. Workflow wraps node-like objects in edges automatically, so the agent can be passed directly.
3. (writer_node, "END") — "END" is not a valid node-like value. Per build_node's docstring the accepted types are BaseNode, BaseAgent, BaseTool, a callable, or 'START', so this raises WorkflowConfigurationError.

Solution:

Pass the agent directly and drop the terminal edge, matching the other workflow guides:

from google.adk.agents import LlmAgent
from google.adk.workflow import Workflow

writer_agent = LlmAgent(
    name="writer",
    instruction="Write a short story about the input topic."
)

wf = Workflow(
    name="story_generator",
    edges=[
        ("START", writer_agent)
    ]
)

This follows the pattern in docs/guides/workflow/graph/index.md, function_node/index.md, base_node/index.md, and contributing/samples/workflows/agent_in_workflow/agent.py, none of which call build_node or use an "END" endpoint.

Testing Plan

Small documentation fix, so per the template this section is not required. For completeness, the verification performed:

1. Parsed src/google/adk/workflow/__init__.py with ast and enumerated its module-level bindings and __all__: build_node is absent, Workflow is present.
2. Confirmed no workflow/build_node.py or workflow/build_node/ submodule exists.
3. Read build_node's docstring, which documents WorkflowConfigurationError for node-like values outside BaseNode/BaseAgent/BaseTool/callable/'START'.
4. Scanned all 1327 from google.adk... import ... statements across docs/, contributing/, and .agents/, resolving submodule imports. This was the last broken import; the count is now zero.