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
• use released Pydantic AI >=2.46.0,<3 and Beartype >=0.23.0,<0.24, with Logfire >=4.39,<5
• document agent-isolation sandboxes as unsupported by this integration

Validation

All 11 Pydantic AI tests pass on Python 3.10 (Pydantic AI 2.54.0) and Python 3.14 (Pydantic AI 2.55.0). Lint, formatting, type checks on both Python versions, and lockfile validation pass.

• 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

[brianstrauch]

We can optionally wait for beartype and pydantic releases before merging so we're not pinning commits