feat: 完善 AI-Core 文档解析器

- 添加多种文档解析器 (PDF, Word, Excel, Markdown 等)
- 添加基础解析器和链式解析器
- 添加存储和注册机制
- 添加 gRPC 服务实现

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ai-core/parser/pdf_parser.py

@@ -0,0 +1,15 @@
+from docreader.parser.chain_parser import FirstParser
+from docreader.parser.markitdown_parser import MarkitdownParser
+
+
+class PDFParser(FirstParser):
+    """PDF Parser using chain of responsibility pattern
+    
+    Attempts to parse PDF files using multiple parser backends in order:
+    1. MinerUParser - Primary parser for PDF documents
+    2. MarkitdownParser - Fallback parser if MinerU fails
+    
+    The first successful parser result will be returned.
+    """
+    # Parser classes to try in order (chain of responsibility pattern)
+    _parser_cls = (MarkitdownParser,)