fix: reset _default_converter on init failure (H5)

If the lazy-init of the default converter fails (e.g. model download
error), the singleton was left as None but subsequent calls would not
retry. Now the failed state is cleared so the next request retries.

Ref #57 (H5)

document-parser/infra/local_converter.py

@@ -118,7 +118,11 @@ def _build_docling_converter(options: ConversionOptions) -> DoclingConverter:
 def _get_default_converter() -> DoclingConverter:
     global _default_converter
     if _default_converter is None:
-        _default_converter = _build_docling_converter(ConversionOptions())
+        try:
+            _default_converter = _build_docling_converter(ConversionOptions())
+        except Exception:
+            _default_converter = None
+            raise
     return _default_converter