fix: strip unsupported JSON Schema fields for Google GenAI provider

When MCP tools provide standard JSON Schema parameters containing
metadata fields like $schema, the google-genai SDK's Pydantic
model rejects them with extra_forbidden validation errors.

Strip unsupported JSON Schema metadata fields ($schema, $id,
$ref, $comment, definitions, $defs) from tool parameters before
passing them to the Google GenAI SDK.

Fixes #734

Author: Br1an67

packages/kosong/src/kosong/contrib/chat_provider/google_genai.py

@@ -354,12 +354,18 @@ def tool_to_google_genai(tool: KosongTool) -> Tool:
     """Convert a Kosong tool to GoogleGenAI tool format."""
     # Kosong already validates parameters as JSON Schema format via jsonschema
     # The google-genai SDK accepts dict format and internally converts to Schema
+    parameters = tool.parameters
+    if isinstance(parameters, dict):
+        # Strip JSON Schema metadata fields that the google-genai SDK's
+        # Pydantic model rejects with extra_forbidden validation errors
+        unsupported_keys = {"$schema", "$id", "$ref", "$comment", "definitions", "$defs"}
+        parameters = {k: v for k, v in parameters.items() if k not in unsupported_keys}
     return Tool(
         function_declarations=[
             FunctionDeclaration(
                 name=tool.name,
                 description=tool.description,
-                parameters=tool.parameters,  # type: ignore[arg-type] # GoogleGenAI accepts dict
+                parameters=parameters,  # type: ignore[arg-type] # GoogleGenAI accepts dict
             )
         ]
     )