fix: pass through JSON Schemas without a "properties" key in create_sdk_mcp_server

[Otis0408]

Summary

tool() documents that input_schema may be "A JSON Schema dictionary for full validation", and create_sdk_mcp_server's _build_schema has a dedicated passthrough branch for it (covered by test_json_schema_dict_passthrough).

But it only recognized a dict as a pre-built JSON Schema when it contained both "type" and "properties". "properties" is optional in JSON Schema — open objects, arrays, and non-object schemas legitimately omit it — so a valid schema without it was misinterpreted as a {param_name: python_type} mapping and mangled:

tool("t", "d", {"type": "object", "additionalProperties": True})
# inputSchema the model sees today:
# {'type': 'object',
#  'properties': {'type': {'type': 'string'},
#                 'additionalProperties': {'type': 'string'}},
#  'required': ['type', 'additionalProperties']}

The schema's own keywords (type, additionalProperties, items, ...) become fake required string parameters, so the model is told the tool needs inputs it doesn't — breaking tool calls. Affected inputs include {"type": "object"}, {"type": "object", "additionalProperties": true} (open/arbitrary-object tools), and {"type": "array", "items": {...}}.

Fix

Detect a JSON Schema by a string-valued type alone; drop the extra "properties" requirement. A {name: type} param mapping never matches this, because its values are Python types — isinstance(input_schema["type"], str) is False even when a param is literally named type (its value is the str type, not the string "object").

Tests

• test_json_schema_without_properties_passthrough — {"type": "object"}, open-object, and array schemas now pass through unchanged (parametrized).
• test_param_mapping_with_param_named_type_still_expands — a mapping with a param named type still expands correctly, guarding the disambiguation the fix relies on.

Verified the passthrough tests fail on main (schema mangled) and pass with the fix; the "param named type" test passes both ways.

1064 passed, 5 skipped
ruff check: All checks passed!
mypy src: Success: no issues found in 24 source files