test(workflow): Move agent transfer integration tests to workflow folder

Migrated the multi-agent dynamic transfer integration tests from `flows/llm_flows/test_agent_transfer.py` to the consolidated `workflow/` directory to align with ADK 2.0 patterns.

During this migration and alignment:
1. Fixed a critical runner bug in `NodeRunner._track_event_in_context` where structured parent nodes (like SequentialAgent or LoopAgent) executing legacy recursive sub-agents internally would mistakenly intercept and bubble up the sub-agent's internal routing actions, causing crashing `unrelated transfer` errors in the parent scheduler. Fixed by restricting route/transfer tracking to native node events (`not event.author` or `event.author == self._node.name`).
2. Restored five previously deleted legacy structured transfer tests (`test_auto_to_single`, `test_auto_to_auto_to_single`, `test_auto_to_sequential`, `test_auto_to_sequential_to_auto`, and `test_auto_to_loop`) to maintain regression protection.
3. Hardened the suite with dedicated new tests for sibling peer transfers, child-to-parent climbing transfers, grandchild nested transfers, self-transfer blocking, and unrelated sandbox crossing defenses.

Change-Id: I2f087c211a5a78508689f4dfeea2c533219df208

Author: DeanChensj

src/google/adk/workflow/_node_runner.py

@@ -285,11 +285,16 @@ def _track_event_in_context(self, event: Event, ctx: Context) -> None:
       ctx.output = event.content
     if event.long_running_tool_ids is not None:
       ctx._interrupt_ids.update(event.long_running_tool_ids)
-    if event.actions.route is not None:
-      ctx.route = event.actions.route
-      ctx._route_emitted = True
-    if event.actions.transfer_to_agent is not None:
-      ctx.actions.transfer_to_agent = event.actions.transfer_to_agent
+    # Only propagate decisions from native events (authored by this node or unspecified).
+    # This prevents structured parent nodes (e.g. SequentialAgent) from intercepting
+    # and bubbling up actions already handled internally by their nested sub-agents.
+    is_native_node_event = not event.author or event.author == self._node.name
+    if event.actions and is_native_node_event:
+      if event.actions.route is not None:
+        ctx.route = event.actions.route
+        ctx._route_emitted = True
+      if event.actions.transfer_to_agent is not None:
+        ctx.actions.transfer_to_agent = event.actions.transfer_to_agent
 
     ctx.telemetry_context.add_event(event)