Migrate to temporalio-openai-agents and add MCP v2 sample

[brianstrauch]

Summary

• replace the Temporal SDK openai-agents extra with the standalone temporalio-openai-agents package
• migrate integration imports from temporalio.contrib.openai_agents to temporalio.openai_agents
• add an isolated openai_agents/mcp_v2 sample using MCP SDK v2 streamable HTTP, named OpenAIAgentsPlugin server factories, and temporal_mcp_server workflow proxies
• retain the existing openai_agents/mcp samples on the MCP v1 compatibility API
• document concise sample-local uv commands and refresh the root and sample lockfiles

Testing

• uv lock --check
• cd openai_agents/mcp_v2, then uv lock --check
• uv run poe lint
• uv run --group openai-agents pytest tests/openai_agents
• import and construct all MCP v2 sample entry points in the isolated locked environment
• start and cleanly stop uv run tools_server.py from openai_agents/mcp_v2