fix: fall back from empty Docling native chunks (#15601)

## Summary
- keep the native Docling chunking path when it returns usable chunks
- fall back to the standard Docling response parser when a chunked
request gets HTTP 200 but returns no usable chunks
- add a regression test for older Docling servers that accept the
chunking request but return a standard conversion payload

## Why
Older external Docling servers can accept a request containing
`do_chunking: true` and still return the standard conversion response
shape. The current code treats any HTTP 200 from the chunked request as
a native chunk response, finds no chunk entries, and returns zero
sections without trying the standard response parser.

Fixes #15569.

## Validation
- `python -m pytest
test\\unit_test\\deepdoc\\parser\\test_docling_parser_remote.py -q`
- `python -m py_compile deepdoc\\parser\\docling_parser.py
test\\unit_test\\deepdoc\\parser\\test_docling_parser_remote.py`
- `python -m ruff check deepdoc\\parser\\docling_parser.py
test\\unit_test\\deepdoc\\parser\\test_docling_parser_remote.py`
- `git diff --check`

deepdoc/parser/docling_parser.py

@@ -477,7 +477,10 @@ class DoclingParser(RAGFlowPdfParser):
                     
             if callback:
                 callback(0.95, f"[Docling] Native chunks received: {len(sections)}")
-            return sections, tables
+            if sections:
+                return sections, tables
+
+            self.logger.warning("[Docling] Native chunking returned no usable chunks; trying standard response parsing.")
 
         # --- FALLBACK: Standard RAGFlow parsing for older docling servers ---
         docs = self._extract_remote_document_entries(response_json)