feat(a2a): add session-aware AgentKit remote skill agent

[zakahan]

Summary

• add AgentkitRemoteSkillAgent, a lazy BaseAgent proxy that resolves the current AgentKit Skill Sandbox Session before creating its RemoteVeAgent delegate
• reuse physical Sessions by a stable logical key and rotate them when their remaining lifetime cannot cover the request timeout and safety buffer
• wait for a valid A2A Agent Card after Session Ready, preserving Session endpoint query parameters and inbound authentication
• isolate A2A task context by physical Session and support polling non-blocking sandbox tasks
• add pagination, concurrency protection, ambiguous CreateSession recovery, and focused unit coverage

Motivation

AgentKit Skill Sandbox Sessions are ephemeral and their A2A endpoint changes after expiration. A normal RemoteVeAgent requires a fixed URL during construction, so it cannot survive Session rotation. This change delays URL binding until invocation time and rebuilds the delegate when the physical Session changes.

Testing

• uv run pre-commit run --files veadk/a2a/agentkit_remote_skill_agent.py veadk/a2a/init.py veadk/a2a/remote_ve_agent.py veadk/tools/builtin_tools/_agentkit.py tests/a2a/test_agentkit_remote_skill_agent.py tests/tools/builtin_tools/test_agentkit.py
• uv run pytest -q tests/a2a tests/tools/builtin_tools/test_agentkit.py tests/tools/builtin_tools/test_run_sandbox_agent.py

Result: 72 passed, 6 subtests passed.