fix(agent_transfer): add hasattr guard for peer agents without mode attribute

[AliMuhammadAslam]

Fixes #5863

---

_get_transfer_targets already uses a hasattr guard when building the sub-agent list:

if not hasattr(sub_agent, 'mode')
or sub_agent.mode not in ('single_turn', 'task')

The peer-agent list comprehension a few lines later was missing the same guard:

if peer_agent.name != agent.name
and peer_agent.mode not in ('single_turn', 'task')  # crashes on LoopAgent

LoopAgent (and any other BaseAgent subclass without a mode attribute) raises AttributeError: 'LoopAgent' object has no attribute 'mode' here, breaking any setup where a LoopAgent appears as a sibling of an LlmAgent under a shared parent.

The fix applies the same not hasattr(peer_agent, 'mode') guard to the peer list, matching the pattern already used for sub-agents.

Testing plan

Regression test added: tests/unittests/flows/llm_flows/test_get_transfer_targets.py

• test_loop_agent_peer_does_not_raise_attribute_error — sets up a root agent with an LlmAgent and a LoopAgent as peer sub-agents, calls _get_transfer_targets on the LlmAgent, and asserts it returns the LoopAgent without raising.
• test_loop_agent_sub_agent_does_not_raise_attribute_error — same check for the sub-agent list path (already worked before; included for symmetry).

Before the fix:

AttributeError: 'LoopAgent' object has no attribute 'mode'
  File ".../agent_transfer.py", line 196, in <listcomp>
    and peer_agent.mode not in ('single_turn', 'task')

After the fix:

# _get_transfer_targets returns normally, LoopAgent included as a peer target

All existing tests in tests/unittests/flows/llm_flows/ continue to pass.

[adk-bot]

Response from ADK Triaging Agent

Hello @AliMuhammadAslam, thank you for creating this PR!

While we appreciate the regression tests included with your change, could you please add a testing plan section in your PR description to describe how you tested or intend to test this?

Additionally, since this is a bug fix, providing some command-line logs or output demonstrating the fix would be extremely helpful for the reviewers.

For more details, please refer to our contribution guidelines. This information will help reviewers review your PR more efficiently. Thanks!

[AliMuhammadAslam]

Updated the PR description with a testing plan and before/after output as requested. The regression tests are in tests/unittests/flows/llm_flows/test_get_transfer_targets.py.