fix(mcp): coerce stringified arrays/objects in tool args

When a tool schema declares `type: array` or `type: object` and the model
emits the value as a JSON string (common with complex oneOf discriminated
unions), the MCP server rejects it with -32602 "expected array, received
string".  Extend `_coerce_value` to attempt `json.loads` for these types
and replace the string with the parsed value before dispatch.

Root cause confirmed via live testing: `add_reminders.reminders` uses a
oneOf discriminated union (relative/absolute/location) that triggers model
output drift.  Sending a real array passes validation; sending a string
reproduces the exact error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

model_tools.py

@@ -418,6 +418,31 @@ def _coerce_value(value: str, expected_type):
         return _coerce_number(value, integer_only=(expected_type == "integer"))
     if expected_type == "boolean":
         return _coerce_boolean(value)
+    if expected_type == "array":
+        return _coerce_json(value, list)
+    if expected_type == "object":
+        return _coerce_json(value, dict)
+    return value
+
+
+def _coerce_json(value: str, expected_python_type: type):
+    """Parse *value* as JSON when the schema expects an array or object.
+
+    Handles model output drift where a complex oneOf/discriminated-union schema
+    causes the LLM to emit the array/object as a JSON string instead of a native
+    structure.  Returns the original string if parsing fails or yields the wrong
+    Python type.
+    """
+    try:
+        parsed = json.loads(value)
+    except (ValueError, TypeError):
+        return value
+    if isinstance(parsed, expected_python_type):
+        logger.debug(
+            "coerce_tool_args: coerced string to %s via json.loads",
+            expected_python_type.__name__,
+        )
+        return parsed
     return value