fix: handle MCP tool schemas missing properties key gracefully (#9347)

MCP tools can have an inputSchema like `{"type": "object", "additionalProperties": false}`
with no `properties` key (valid for zero-arg tools). The hard assert on `"properties" in
parameters_schema` caused an AssertionError. Replace assertions with safe defaults.

Datadog: https://us5.datadoghq.com/error-tracking/issue/ecee9ee2-db75-11f0-ba66-da7ad0900000

🤖 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/functions/schema_generator.py

@@ -704,8 +704,9 @@ def generate_tool_schema_for_mcp(
     name = mcp_tool.name
     description = mcp_tool.description
 
-    assert "type" in parameters_schema, parameters_schema
-    assert "properties" in parameters_schema, parameters_schema
+    if "type" not in parameters_schema:
+        parameters_schema["type"] = "object"
+    parameters_schema.setdefault("properties", {})
     # assert "required" in parameters_schema, parameters_schema
 
     # Normalize the schema to fix common issues with MCP schemas