refactor: 重构 Reader 内部工具函数到独立模块

- 新增 scripts/readers/_utils.py 作为 Reader 内部共享工具模块
- 将 parse_with_markitdown 等函数从 core/markdown.py 迁移到 _utils.py
- 函数重命名：parse_with_xxx → parse_via_xxx，_unstructured_elements_to_markdown → convert_unstructured_to_markdown
- 更新 17 个 Reader 实现文件的 import 路径
- 从 core/__init__.py 移除已迁移函数的导出
- 新增测试文件 tests/test_readers/test_utils.py
- 新增 spec 文档 openspec/specs/reader-internal-utils/spec.md

这次重构明确了模块边界：core/ 提供公共 API，readers/_utils.py 提供 Reader 内部工具

tests/test_readers/test_utils.py

@@ -0,0 +1,203 @@
+"""测试 Reader 内部工具函数。"""
+
+import zipfile
+import pytest
+from scripts.readers._utils import (
+    parse_via_markitdown,
+    parse_via_docling,
+    build_markdown_table,
+    flush_list_stack,
+    safe_open_zip,
+    convert_unstructured_to_markdown,
+    _UNSTRUCTURED_RGB_PATTERN,
+    _UNSTRUCTURED_PAGE_NUMBER_PATTERN,
+)
+
+
+class TestBuildMarkdownTable:
+    """测试 build_markdown_table 函数。"""
+
+    def test_standard_table(self):
+        """测试标准表格格式化。"""
+        rows_data = [["姓名", "年龄"], ["张三", "25"], ["李四", "30"]]
+        result = build_markdown_table(rows_data)
+
+        assert "| 姓名 | 年龄 |" in result
+        assert "| --- | --- |" in result
+        assert "| 张三 | 25 |" in result
+        assert "| 李四 | 30 |" in result
+
+    def test_empty_table(self):
+        """测试空表格。"""
+        assert build_markdown_table([]) == ""
+        assert build_markdown_table([[]]) == ""
+
+    def test_table_with_empty_cells(self):
+        """测试包含空单元格的表格。"""
+        rows_data = [["A", "B"], ["", "C"], ["D", ""]]
+        result = build_markdown_table(rows_data)
+
+        assert "| A | B |" in result
+        assert "|  | C |" in result
+        assert "| D |  |" in result
+
+
+class TestFlushListStack:
+    """测试 flush_list_stack 函数。"""
+
+    def test_flush_non_empty_items(self):
+        """测试刷新非空堆栈。"""
+        list_stack = ["item1\n", "", "item2\n"]
+        target = []
+
+        flush_list_stack(list_stack, target)
+
+        assert target == ["item1\n\n", "item2\n\n"]
+        assert list_stack == []
+
+    def test_flush_all_empty(self):
+        """测试刷新空堆栈。"""
+        list_stack = ["", "", ""]
+        target = []
+
+        flush_list_stack(list_stack, target)
+
+        assert target == []
+        assert list_stack == []
+
+
+class TestSafeOpenZip:
+    """测试 safe_open_zip 函数。"""
+
+    def test_open_valid_file(self, tmp_path):
+        """测试打开合法文件。"""
+        # 创建测试 ZIP 文件
+        zip_path = tmp_path / "test.zip"
+        with zipfile.ZipFile(zip_path, "w") as zf:
+            zf.writestr("valid.txt", "content")
+
+        with zipfile.ZipFile(zip_path, "r") as zf:
+            result = safe_open_zip(zf, "valid.txt")
+            assert result is not None
+            assert result.read() == b"content"
+
+    def test_reject_path_traversal(self, tmp_path):
+        """测试拒绝路径遍历攻击。"""
+        zip_path = tmp_path / "test.zip"
+        with zipfile.ZipFile(zip_path, "w") as zf:
+            zf.writestr("safe.txt", "content")
+
+        with zipfile.ZipFile(zip_path, "r") as zf:
+            assert safe_open_zip(zf, "../etc/passwd") is None
+            assert safe_open_zip(zf, "sub/../../etc/passwd") is None
+
+    def test_reject_absolute_path(self, tmp_path):
+        """测试拒绝绝对路径。"""
+        zip_path = tmp_path / "test.zip"
+        with zipfile.ZipFile(zip_path, "w") as zf:
+            zf.writestr("safe.txt", "content")
+
+        with zipfile.ZipFile(zip_path, "r") as zf:
+            assert safe_open_zip(zf, "/absolute/path.txt") is None
+            assert safe_open_zip(zf, "C:\\Windows\\System32\\config") is None
+
+    def test_empty_name(self):
+        """测试空文件名。"""
+        import io
+
+        zip_buffer = io.BytesIO()
+        with zipfile.ZipFile(zip_buffer, "w") as zf:
+            zf.writestr("test.txt", "content")
+
+        with zipfile.ZipFile(io.BytesIO(zip_buffer.getvalue()), "r") as zf:
+            assert safe_open_zip(zf, "") is None
+
+
+class TestUnstructuredPatterns:
+    """测试 unstructured 噪声匹配模式。"""
+
+    def test_rgb_pattern(self):
+        """测试 RGB 颜色值模式。"""
+        assert _UNSTRUCTURED_RGB_PATTERN.match("R:255 G:128 B:0")
+        assert _UNSTRUCTURED_RGB_PATTERN.match("R:0 G:0 B:0")
+        assert _UNSTRUCTURED_RGB_PATTERN.match("R:255 G:255 B:255")
+
+    def test_rgb_pattern_invalid(self):
+        """测试无效 RGB 值。"""
+        assert not _UNSTRUCTURED_RGB_PATTERN.match("255 128 0")
+        assert not _UNSTRUCTURED_RGB_PATTERN.match("RGB:255 G:128 B:0")
+
+    def test_page_number_pattern(self):
+        """测试页码模式。"""
+        assert _UNSTRUCTURED_PAGE_NUMBER_PATTERN.match("— 3 —")
+        assert _UNSTRUCTURED_PAGE_NUMBER_PATTERN.match("— 123 —")
+        assert _UNSTRUCTURED_PAGE_NUMBER_PATTERN.match("— 1 —")
+
+    def test_page_number_pattern_invalid(self):
+        """测试无效页码。"""
+        assert not _UNSTRUCTURED_PAGE_NUMBER_PATTERN.match("Page 3")
+        assert not _UNSTRUCTURED_PAGE_NUMBER_PATTERN.match("--- 3 ---")
+
+
+class TestConvertUnstructuredToMarkdown:
+    """测试 convert_unstructured_to_markdown 函数。"""
+
+    def test_skip_rgb_pattern(self):
+        """测试跳过 RGB 噪声。"""
+        try:
+            from unstructured.documents.elements import Text
+        except ImportError:
+            pytest.skip("unstructured 库未安装")
+
+        elements = [Text("R:255 G:128 B:0"), Text("正常文本")]
+        result = convert_unstructured_to_markdown(elements)
+
+        assert "R:255 G:128 B:0" not in result
+        assert "正常文本" in result
+
+    def test_skip_page_number_pattern(self):
+        """测试跳过页码噪声。"""
+        try:
+            from unstructured.documents.elements import Text
+        except ImportError:
+            pytest.skip("unstructured 库未安装")
+
+        elements = [Text("— 3 —"), Text("正常文本")]
+        result = convert_unstructured_to_markdown(elements)
+
+        assert "— 3 —" not in result
+        assert "正常文本" in result
+
+    def test_convert_without_markdownify(self):
+        """测试未安装 markdownify 时的回退行为。"""
+        # 创建简单的 mock 对象
+        class MockElement:
+            def __init__(self, text):
+                self.text = text
+
+        elements = [MockElement("文本1"), MockElement("文本2")]
+        result = convert_unstructured_to_markdown(elements)
+
+        # 应该回退到简单连接文本
+        assert "文本1" in result
+        assert "文本2" in result
+
+
+class TestParseViaMarkitdown:
+    """测试 parse_via_markitdown 函数。"""
+
+    def test_parse_nonexistent_file(self):
+        """测试解析不存在的文件。"""
+        content, error = parse_via_markitdown("/nonexistent/file.txt")
+        assert content is None
+        assert error is not None
+
+
+class TestParseViaDocling:
+    """测试 parse_via_docling 函数。"""
+
+    def test_parse_nonexistent_file(self):
+        """测试解析不存在的文件。"""
+        content, error = parse_via_docling("/nonexistent/file.txt")
+        assert content is None
+        assert error is not None