Add Pydantic AI plugin samples

[brianstrauch]

Summary

• add runnable Pydantic AI Temporal examples for chat, tools, stateless MCP, Workflow Streams, human approval, structured output, multi-agent handoff, and Logfire observability
• provide credential-free TestModel execution plus an optional Pydantic AI Gateway live mode
• register agents declaratively through PydanticAIWorkflow and PydanticAIPlugin
• pin pydantic-ai PR #6639 for AgentEventStream and stream_agent_events support
• document agent-isolation sandboxes as unsupported by this integration

Validation

• uv run --group pydantic-ai pytest -q tests/pydantic_ai_plugin
• uv run --group pydantic-ai ruff check pydantic_ai_plugin tests/pydantic_ai_plugin
• uv run --group pydantic-ai ruff format --check pydantic_ai_plugin tests/pydantic_ai_plugin
• uv run --group pydantic-ai mypy --check-untyped-defs --namespace-packages pydantic_ai_plugin tests/pydantic_ai_plugin
• uv lock --check