fix(deps): require mcp>=1.19 for CallToolResult passthrough

[jujumilk3]

Since #717, create_sdk_mcp_server() returns a CallToolResult from its @server.call_tool() handler. The mcp low-level server only has the isinstance(results, CallToolResult) passthrough branch starting at mcp==1.19.0. On mcp<=1.18.x, CallToolResult (a pydantic BaseModel) falls into the iterable branch, yielding (field, value) tuples that fail ContentBlock union validation with 20 errors (4 fields x 5 union members).

Bump the minimum mcp constraint so every install has the passthrough branch #717 relies on.

mcp<1.19.0 makes every SDK MCP tool call fail with:

pydantic_core._pydantic_core.ValidationError: 20 validation errors for CallToolResult
content.0.TextContent
  Input should be a valid dictionary or instance of TextContent
  [type=model_type, input_value=('meta', None), input_type=tuple]
content.1.TextContent
  ... input_value=('content', [TextContent(...)]) ...
content.2.TextContent
  ... input_value=('structuredContent', None) ...
content.3.TextContent
  ... input_value=('isError', False) ...

The four tuples are exactly the four fields of CallToolResult in declaration order (meta, content, structuredContent, isError) — the fingerprint of a pydantic BaseModel being iterated as a list.

Root cause

The mcp low-level server's result-normalization dispatch lives in mcp/server/lowlevel/server.py::call_tool. From mcp>=1.19.0 it starts with:

if isinstance(results, types.CallToolResult):
    return types.ServerResult(results)
elif isinstance(results, tuple) and len(results) == 2:
    ...
elif isinstance(results, dict):
    ...
elif hasattr(results, "__iter__"):
    unstructured_content = cast(UnstructuredContent, results)

In mcp<=1.18.x the first isinstance(results, CallToolResult) branch does not exist. A CallToolResult is a pydantic BaseModel, which:

• fails isinstance(_, tuple) ✗
• fails isinstance(_, dict) ✗
• passes hasattr(_, "__iter__") ✓ (pydantic models yield (field, value) tuples)

So mcp<=1.18 falls into the iterable branch, does content=list(unstructured_content) → [('meta', None), ('content', [TextContent(...)]), ('structuredContent', None), ('isError', False)], and re-wraps those tuples in CallToolResult(content=...), triggering 20 ContentBlock union errors (4 tuples × 5 union members).

#717's description said "The MCP framework already checks isinstance(results, CallToolResult) and passes it through as-is (confirmed in MCP server source)" — which is correct, but only from mcp==1.19.0 onward. The min-version bump was missed.

mcp versions checked

Downloaded each sdist from PyPI and grepped src/mcp/server/lowlevel/server.py for the isinstance(results, types.CallToolResult) branch:

version	passthrough branch?
1.13.1	❌
1.14.0	❌
1.18.0	❌
1.19.0	✅ (first version with the branch)
1.20.0+	✅