Change return type of _serialize_text_object_value to str

Co-authored-by: Kevin A. Mitchell <kam@datalogics.com>

Author: datalogics-cgreen

src/pdfrest/models/_internal.py

@@ -148,7 +148,7 @@ def _serialize_redactions(value: list[_PdfRedactionVariant]) -> str:
     return json.dumps(payload, separators=(",", ":"))
 
 
-def _serialize_text_object_value(value: Any) -> Any:
+def _serialize_text_object_value(value: Any) -> str:
     if isinstance(value, str):
         return value
     return json.dumps(value, separators=(",", ":"))