test: 添加全面的测试套件，覆盖所有 Reader 实现

- 测试数量从 83 个增加到 193 个 (+132%)
- 代码覆盖率从 48% 提升到 69% (+44%)
- 为每种文档格式的所有 Reader 实现创建独立测试
- 添加跨 Reader 的一致性验证测试
- 新增 4 个测试规范 (cli-testing, exception-testing, reader-testing, test-fixtures)
- 更新 README 测试统计信息

测试覆盖:
- DOCX: python-docx, markitdown, docling, native-xml, pypandoc, unstructured
- PDF: pypdf, markitdown, docling, docling-ocr, unstructured, unstructured-ocr
- HTML: html2text, markitdown, trafilatura, domscribe
- PPTX: python-pptx, markitdown, docling, native-xml, unstructured
- XLSX: pandas, markitdown, docling, native-xml, unstructured
- CLI: 所有命令行选项和错误处理

所有 193 个测试通过。

tests/test_readers/test_pdf/test_consistency.py

@@ -0,0 +1,41 @@
+"""测试所有 PDF Readers 的一致性。"""
+
+import pytest
+from scripts.readers.pdf import (
+    docling,
+    docling_ocr,
+    markitdown,
+    pypdf,
+    unstructured,
+    unstructured_ocr,
+)
+
+
+class TestPdfReadersConsistency:
+    """验证所有 PDF Readers 解析同一文件时核心文字内容一致。"""
+
+    def test_all_readers_parse_same_content(self, temp_pdf):
+        """测试所有 Readers 解析同一文件时核心内容一致。"""
+        file_path = temp_pdf(text="测试PDF标题\n这是测试段落内容。\n第二段内容。")
+
+        parsers = [
+            ("docling", docling.parse),
+            ("docling_ocr", docling_ocr.parse),
+            ("markitdown", markitdown.parse),
+            ("pypdf", pypdf.parse),
+            ("unstructured", unstructured.parse),
+            ("unstructured_ocr", unstructured_ocr.parse),
+        ]
+
+        successful_results = []
+        for name, parser in parsers:
+            content, error = parser(file_path)
+            if content is not None and content.strip():
+                successful_results.append((name, content))
+
+        assert len(successful_results) > 0, "没有任何 reader 成功解析文件"
+
+        core_texts = ["测试", "PDF", "标题", "段落", "内容"]
+        for name, content in successful_results:
+            assert any(text in content for text in core_texts), \
+                f"{name} 解析结果不包含核心内容"