some fixes again

python_app/models/run_config_validation.py

@@ -30,6 +30,8 @@ def _require_int(payload: dict[str, Any], key: str, default: int) -> int:
     config-error contract; surface it as a ValueError naming the field instead.
     """
     value = payload.get(key, default)
+    if value is None:  # explicit JSON null -> use the default, never coerce
+        return default
     if isinstance(value, bool) or not isinstance(value, (int, float, str)):
         raise ValueError(f"{key} must be a JSON integer")
     try:
@@ -41,6 +43,8 @@ def _require_int(payload: dict[str, Any], key: str, default: int) -> int:
 def _require_str(payload: dict[str, Any], key: str, default: str) -> str:
     """Read a string field, rejecting JSON arrays/objects with a named ValueError."""
     value = payload.get(key, default)
+    if value is None:  # explicit JSON null -> use the default, never coerce to "None"
+        return default
     if isinstance(value, (dict, list)):
         raise ValueError(f"{key} must be a JSON string")
     return str(value)
@@ -49,6 +53,8 @@ def _require_str(payload: dict[str, Any], key: str, default: str) -> str:
 def _require_bool(payload: dict[str, Any], key: str, default: bool) -> bool:
     """Read a boolean field, rejecting non-boolean JSON types with a named ValueError."""
     value = payload.get(key, default)
+    if value is None:  # explicit JSON null -> use the default
+        return default
     if not isinstance(value, bool):
         raise ValueError(f"{key} must be a JSON boolean")
     return value