fix(pydantic): support pydantic-ai v2 in restate.ext.pydantic while keeping v1

[selimacerbas]

Problem

restate.ext.pydantic is written against pydantic-ai v1 and crashes on import under
pydantic-ai-slim==2.0.0:

>>> from restate.ext.pydantic import RestateAgent
ModuleNotFoundError: No module named 'pydantic_ai.builtin_tools'

The pydantic_ai optional dependency is declared open-ended (pydantic-ai-slim>=1.68.0),
so 2.0.0 resolves and then fails at import time.

What changed (v1 1.68+ AND v2 2.0.0 both supported)

v1 → v2 change	Handling (kept v1-compatible)
pydantic_ai.builtin_tools deleted → pydantic_ai.native_tools; AbstractBuiltinTool → AbstractNativeTool, tools.BuiltinToolFunc → tools.NativeToolFunc	try the v2 names, except ImportError fall back to the v1 module aliased to the same local names. (v1 1.68+ already exposes the new names, so no deprecation warnings on v1.)
pydantic_ai.mcp.MCPServer renamed to pydantic_ai.mcp.MCPToolset — separate classes: 1.x concrete servers subclass MCPServer, 2.0 only has MCPToolset	Detect against the tuple of MCP base classes present in the installed version (MCP_TOOLSET_CLASSES); isinstance accepts a tuple. A single aliased name would silently stop wrapping MCP servers on whichever line it didn't match.
Agent.run(builtin_tools=…) and its **_deprecated_kwargs shim removed in v2; native tools now go via capabilities=[NativeTool(tool), …]	RestateAgent.run translates builtin_tools → capabilities=[NativeTool(t) …] when the NativeTool capability is available (1.68+), and never forwards builtin_tools on v2; oldest 1.x falls back to forwarding builtin_tools=.

The RestateAgent(event_stream_handler=…) public API is unchanged. The handler was only
ever passed at run time (still accepted in v2) and stored on RestateModelWrapper, never
to a pydantic-ai constructor — so RestateModelWrapper.request_stream continues to read
self._event_stream_handler exactly as before. (For reference, v2's
capabilities.ProcessEventStream(handler=…) is the new way to register a handler on the
agent, and its docstring confirms the run-time event_stream_handler parameter still
observes live events under durable-execution integrations — so no change is needed here.)

Repro

uv add restate-sdk 'pydantic-ai-slim[openai,mcp]==2.0.0'
python -c "from restate.ext.pydantic import RestateAgent"   # before: ModuleNotFoundError

The break is iterative (builtin_tools module → BuiltinToolFunc → MCPServer), plus a
runtime-only TypeError on the first agent.run() from forwarding builtin_tools= to a
base run() that no longer accepts it.

Tests

Verified on both pydantic-ai-slim==1.107.0 and ==2.0.0 against this branch:

• import restate.ext.pydantic (clean, no deprecation warnings on v1)
• RestateAgent(inner, event_stream_handler=handler) construction; handler flows into
  RestateModelWrapper._event_stream_handler and the event_stream_handler property
• RestateAgent.run(builtin_tools=[WebSearchTool()]) translates to
  capabilities=[NativeTool(...)] and forwards no unknown kwargs (validated against the
  real base run() signature)
• A real MCP server (MCPServerStdio on v1, MCPToolset on v2) is wrapped as
  RestateMCPServer

ruff clean; pyright introduces no new errors (removes the v2-incompatibility ones).

There is currently no test fixture for restate.ext.pydantic in the repo — happy to
contribute the two smoke tests above + a pydantic-ai 2.x CI matrix entry as a follow-up if
that's useful.

Follow-up to #201.

[gvdongen]

Hi! Thank you for this PR. I think we don't want to keep supporting compatibility with older versions of Pydantic AI. So I'll migrate the SDK to Pydantic AI 2.0 and drop support for 1.x from the next Restate SDK release onwards. Hope this works for you?

[selimacerbas]

Hey @gvdongen, thanks for your answer, of course it works for me, do you want me complete this PR removing v1 support, along with tests etc?

[gvdongen]

@selimacerbas I will take it from here because there is anyway some other stuff I wanted to do. But thanks a lot getting things started! I'll let you know once it's out