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_docx/test_unstructured_docx.py

@@ -0,0 +1,53 @@
+"""测试 Unstructured DOCX Reader 的解析功能。"""
+
+import pytest
+import os
+from scripts.readers.docx import unstructured
+
+
+class TestUnstructuredDocxReaderParse:
+    """测试 Unstructured DOCX Reader 的 parse 方法。"""
+
+    def test_normal_file(self, temp_docx):
+        """测试正常 DOCX 文件解析。"""
+        file_path = temp_docx(
+            headings=[(1, "主标题"), (2, "子标题")],
+            paragraphs=["这是第一段内容。", "这是第二段内容。"],
+            table_data=[["列1", "列2"], ["数据1", "数据2"]],
+            list_items=["列表项1", "列表项2"]
+        )
+
+        content, error = unstructured.parse(file_path)
+
+        if content is not None:
+            assert "主标题" in content or "子标题" in content or "第一段内容" in content
+
+    def test_file_not_exists(self, tmp_path):
+        """测试文件不存在的情况。"""
+        non_existent_file = str(tmp_path / "non_existent.docx")
+        content, error = unstructured.parse(non_existent_file)
+        assert content is None
+        assert error is not None
+
+    def test_empty_file(self, temp_docx):
+        """测试空 DOCX 文件。"""
+        file_path = temp_docx()
+        content, error = unstructured.parse(file_path)
+        assert content is None or content.strip() == ""
+
+    def test_corrupted_file(self, temp_docx, tmp_path):
+        """测试损坏的 DOCX 文件。"""
+        file_path = temp_docx(paragraphs=["测试内容"])
+        with open(file_path, "wb") as f:
+            f.write(b"corrupted content")
+        content, error = unstructured.parse(file_path)
+        assert content is None
+        assert error is not None
+
+    def test_special_chars(self, temp_docx):
+        """测试特殊字符处理。"""
+        special_texts = ["中文测试内容", "Emoji测试: 😀🎉🚀", "特殊符号: ©®™°±"]
+        file_path = temp_docx(paragraphs=special_texts)
+        content, error = unstructured.parse(file_path)
+        if content is not None:
+            assert "中文测试内容" in content or "😀" in content