refactor(parser): reorganize Markdown parser and enhance gRPC document reading

- Moved the _SEPARATOR_CELL regex definition to a more appropriate location in the Markdown parser.
- Implemented a fallback mechanism in the gRPC document reader to handle cases where the ReadStream RPC is unimplemented, ensuring compatibility with older versions.
- Added a readUnary method to maintain backward compatibility with the legacy unary Read RPC.
- Improved cancellation handling in the MinerUCloud and PaddleOCR-VL readers to prevent excessive API calls during context cancellation.

docreader/parser/markdown_parser.py

@@ -18,8 +18,6 @@ import re
 import uuid
 from typing import Dict, List, Match, Optional, Tuple
 
-_SEPARATOR_CELL = re.compile(r"^:?-{3,}:?$")
-
 from docreader.models.document import Document
 from docreader.parser.base_parser import BaseParser
 from docreader.parser.chain_parser import PipelineParser
@@ -28,6 +26,8 @@ from docreader.utils import endecode
 # Get logger object
 logger = logging.getLogger(__name__)
 
+_SEPARATOR_CELL = re.compile(r"^:?-{3,}:?$")
+
 
 class MarkdownTableUtil:
     """Utility class for formatting Markdown tables.