fix: classify TransferToAgentTool transfers to RemoteA2aAgent as TRANSFER_A2A

[caohy1988]

Summary

Fixes #5073

When a RemoteA2aAgent is registered via sub_agents=[], the BigQueryAgentAnalyticsPlugin now correctly classifies transfers to it as TRANSFER_A2A instead of the generic TRANSFER_AGENT.

Implementation Approach

Per the architectural decision in the issue thread, the fix is localized entirely to the BigQuery analytics plugin — no changes to TransferToAgentTool or the ADK core.

The plugin resolves the transfer origin at call-level in before_tool_callback / after_tool_callback:

1. Detects TransferToAgentTool instances
2. Extracts agent_name from tool_args
3. Looks up the target in the agent tree via tool_context._invocation_context.agent
4. Classifies as TRANSFER_A2A if the target is a RemoteA2aAgent, otherwise TRANSFER_AGENT

This means a single TransferToAgentTool with mixed local and A2A targets produces the correct label per invocation.

Changes

• bigquery_agent_analytics_plugin.py: Added _find_transfer_target() helper and extended _get_tool_origin() with optional tool_args/tool_context parameters for call-level transfer classification
• test_bigquery_agent_analytics_plugin.py: Added 5 new test cases covering A2A sub-agent, local sub-agent, A2A peer, mixed-target per-call classification, and fallback without args

Classification Table

Tool path	Selected target	Label
AgentTool(local agent)	—	SUB_AGENT (unchanged)
AgentTool(RemoteA2aAgent)	—	A2A (unchanged)
FunctionTool	—	LOCAL (unchanged)
McpTool	—	MCP (unchanged)
TransferToAgentTool	local agent	TRANSFER_AGENT (unchanged)
TransferToAgentTool	RemoteA2aAgent	TRANSFER_A2A (new)

Test plan

• All 198 existing plugin tests pass
• 5 new tests cover the new classification:
  • Transfer to RemoteA2aAgent sub-agent → TRANSFER_A2A
  • Transfer to local sub-agent → TRANSFER_AGENT
  • Transfer to RemoteA2aAgent peer → TRANSFER_A2A
  • Mixed targets classify correctly per call
  • Fallback to TRANSFER_AGENT when no tool_args/tool_context provided

🤖 Generated with Claude Code

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