test(samples): add e2e validation script for TRANSFER_A2A classification

Spins up an in-process A2A server (ASGI transport), creates an
orchestrator with one local sub-agent and one RemoteA2aAgent,
runs transfers through BigQueryAgentAnalyticsPlugin with real GCP
credentials, and queries BQ to verify tool_origin correctly shows
TRANSFER_A2A for remote agents and TRANSFER_AGENT for local agents.

Validated against test-project-0728-467323.adk_logs.transfer_a2a_validation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: caohy1988