LCORE-2914: refactor A2A executor to use pydantic-ai agents

Replace direct client.responses.create() calls with build_agent() +
agent.run_stream_events(), aligning the A2A server path with the
query and streaming query endpoints. This enables pydantic-ai skills
and capabilities in A2A-served requests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: are-ces