restate.ext.pydantic requires pydantic-ai-slim[mcp] because _toolset.py imports pydantic_ai.mcp unguarded

[touhonoob]

restate.ext.pydantic can't be imported without pydantic-ai-slim[mcp], even with no MCP toolsets.

What happens

restate/ext/pydantic/__init__.py imports ._toolset, and _toolset.py:14 does an unguarded

from pydantic_ai.mcp import MCPToolset

pydantic_ai/mcp.py raises ImportError at import time when the mcp / fastmcp packages are absent, so with restate-sdk[pydantic-ai] + plain pydantic-ai-slim the whole extension fails to import:

$ pip install 'restate-sdk[pydantic-ai]==1.0.3' 'pydantic-ai-slim==2.12.0'
$ python -c "import restate.ext.pydantic"
ImportError: Please install the `mcp` package to use `MCPToolset`, you can use the `mcp` optional group — pip install "pydantic-ai-slim[mcp]"

Why it looks unintended

_agent.py already treats the same import as optional:

try:
    from pydantic_ai.mcp import MCPToolset
    from ._toolset import RestateMCPToolset
except ImportError:
    pass
else:
    if isinstance(toolset, MCPToolset):
        return RestateMCPToolset(toolset, run_options)

…but that guard can't help, because _agent.py unconditionally imports ._toolset a few lines earlier, and _toolset.py pulls pydantic_ai.mcp at module scope for RestateMCPToolset.

Impact

Anyone using RestateAgent with function tools only has to install pydantic-ai-slim[mcp] — 28 extra transitive packages in our case (fastmcp-slim, mcp, authlib, keyring, jsonschema, sse-starlette, secretstorage, …) — purely to satisfy an import for a class they never instantiate.

Suggested fix

Move RestateMCPToolset (and its MCPToolset import) into its own module imported lazily, or guard the import in _toolset.py the way _agent.py does and define RestateMCPToolset only when it succeeds.

Environment

restate-sdk 1.0.3, pydantic-ai-slim 2.12.0, Python 3.14.