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