Fix TypeError for MCP tools with nullable JSON Schema params

MCP servers (e.g. Zapier) return JSON Schema union types like
`"type": ["string", "null"]` for nullable parameters. The existing
`validate_params()` and `cast_params()` methods expected only simple
strings as `type`, causing `TypeError: unhashable type: 'list'` on
every MCP tool call with nullable parameters.

Add `_resolve_type()` helper that extracts the first non-null type
from union types, and use it in `_cast_value()` and `_validate()`.
Also handle `None` values correctly when the schema declares a
nullable type.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nanobot/agent/tools/base.py

@@ -21,6 +21,20 @@ class Tool(ABC):
         "object": dict,
     }
 
+    @staticmethod
+    def _resolve_type(t: Any) -> str | None:
+        """Resolve JSON Schema type to a simple string.
+
+        JSON Schema allows ``"type": ["string", "null"]`` (union types).
+        We extract the first non-null type so validation/casting works.
+        """
+        if isinstance(t, list):
+            for item in t:
+                if item != "null":
+                    return item
+            return None
+        return t
+
     @property
     @abstractmethod
     def name(self) -> str:
@@ -78,7 +92,7 @@ class Tool(ABC):
 
     def _cast_value(self, val: Any, schema: dict[str, Any]) -> Any:
         """Cast a single value according to schema."""
-        target_type = schema.get("type")
+        target_type = self._resolve_type(schema.get("type"))
 
         if target_type == "boolean" and isinstance(val, bool):
             return val
@@ -131,7 +145,11 @@ class Tool(ABC):
         return self._validate(params, {**schema, "type": "object"}, "")
 
     def _validate(self, val: Any, schema: dict[str, Any], path: str) -> list[str]:
-        t, label = schema.get("type"), path or "parameter"
+        raw_type = schema.get("type")
+        nullable = isinstance(raw_type, list) and "null" in raw_type
+        t, label = self._resolve_type(raw_type), path or "parameter"
+        if nullable and val is None:
+            return []
         if t == "integer" and (not isinstance(val, int) or isinstance(val, bool)):
             return [f"{label} should be integer"]
         if t == "number" and (