fix(runners): fall back to root agent when a resumed call author is not in the tree

`Runner._find_agent_to_run` is annotated `-> BaseAgent` and documents that it
"falls back to root agent if no suitable agents are found". But on a resumable
resume from a function response it returned
`root_agent.find_agent(event.author)` unconditionally, and `find_agent` returns
None when `event.author` is not a node in the agent tree (host integrations
commonly hydrate resumed or external events with author='user'). That None
then flowed into `build_node(agent_to_run)` in `run_async`, raising
"ValueError: Invalid node type: <class 'NoneType'>".

Fix: in the resumable branch, only short-circuit when `find_agent` resolves the
author; otherwise fall through to the existing event scan and root-agent
fallback. This matches the non-resumable branch and the method's documented
contract, so the change only turns a guaranteed crash into the documented
fallback.

Also de-nests the TestRunnerFindAgentToRun tests, which were accidentally
defined inside a module-level test function and never collected, and adds two
regression tests.

Change-Id: I7d64187be5e6443a185a20b0ad08bea3ae017fb1

Author: ykumaraswamy

src/google/adk/runners.py

@@ -1728,7 +1728,15 @@ def _find_agent_to_run(
     # shouldn't trap the next turn on that same agent if it's not transferable.
     # Falling through allows it to return to root.
     if event and event.author and is_resumable:
-      return root_agent.find_agent(event.author)
+      # `find_agent` returns None when the author does not correspond to any
+      # agent in the current hierarchy (e.g. the author is "user" or a stale or
+      # foreign agent name carried over from a previous turn/session). Returning
+      # None here would propagate to `build_node`, raising a confusing
+      # "Invalid node type: <class 'NoneType'>" error. Fall through to the
+      # event-scan logic below (which ultimately falls back to the root agent)
+      # whenever the author cannot be resolved.
+      if (resumed_agent := root_agent.find_agent(event.author)) is not None:
+        return resumed_agent
 
     def _event_filter(event: Event) -> bool:
       """Filters out user-authored events and agent state change events."""