fix(runners): widen Runner.__init__ agent param to accept BaseNode

[anxkhn]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Runner.__init__ 'agent' param typed Optional[BaseAgent] but class field and runtime accept BaseNode (e.g. Workflow) roots #6270

Problem:

The Runner.agent class field is typed Optional[BaseAgent | 'BaseNode']
(runners.py:158) and the runtime explicitly branches on
isinstance(self.agent, BaseNode) in both run_async and run_live, so a
BaseNode root (for example a Workflow) is fully supported at execution time.

However, the Runner.__init__ agent parameter (runners.py:180) was still
typed Optional[BaseAgent], narrower than the field it initializes. As a result,
static type checkers reject Runner(agent=my_workflow) even though it runs
correctly. The parameter annotation was simply out of sync with the class field
and with the runtime's own BaseNode handling.

This is a static typing correctness issue, not a runtime one (Python does not
enforce annotations at runtime, so the call already works). It shows up in ADK's
own codebase: with the current annotation, mypy flags the internal call site
in a2a/utils/agent_to_a2a.py where a plain BaseAgent is forwarded into
Runner:

src/google/adk/a2a/utils/agent_to_a2a.py:169: error: Argument 1 to "Runner"
  has incompatible type ...; expected "BaseAgent | None"  [arg-type]

Solution:

Widen the __init__ agent annotation to Optional[BaseAgent | 'BaseNode'] so
it matches the class field byte-for-byte. This mirrors the existing
forward-reference style already used for the field (runners.py:158) and for the
run_live node parameter (runners.py:455); BaseNode is kept as a lazy
forward reference (the module uses from __future__ import annotations and
imports BaseNode inside the methods that need it), so no new import is added.

This is a type-only change with no runtime behavior change. The narrower internal
helper _resolve_app was intentionally left as-is: this PR is scoped to the
public __init__ surface that #6270 reports.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added TestRunnerResolveApp::test_resolve_app_with_base_node_via_agent_param,
which constructs Runner(agent=<BaseNode>, app_name=...) and asserts the node is
accepted and wrapped (runner.app.root_agent is node, runner.agent is node).
This is a behavioral contract guard for the accepted-and-wrapped path; the
annotation itself is validated by the type checker (below), since Python does not
enforce annotations at runtime.

pytest summary:

$ pytest tests/unittests/test_runners.py -q
70 passed

Type check (the actual validation of a type-only change): with this change,
mypy no longer reports the expected "BaseAgent | None" error at the
a2a/utils/agent_to_a2a.py Runner(...) call site, and no new mypy errors are
introduced elsewhere.

Manual End-to-End (E2E) Tests:

Not applicable. This is a type-annotation-only change with no runtime behavior
change; it is covered by the unit test and the static type check above.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas. (No comment needed; one-line annotation change mirroring the existing field.)
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end. (Not applicable; type-only change.)
• Any dependent changes have been merged and published in downstream modules. (None.)

Additional context

The change is a single line plus one test:

-      agent: Optional[BaseAgent] = None,
+      agent: Optional[BaseAgent | 'BaseNode'] = None,