fix(deepdoc): attach lone header lines to the following section when delimiter is set (#16109)

## Summary
Fixes #15487 — lone markdown headers are no longer isolated as empty
chunks when a custom `delimiter` is set.

- Merge consecutive lone headers before attaching to the following prose
body
- Skip code fences, tables, lists, and blockquotes via
`_is_attachable_body()`
- Unit tests include the `# Title / ## Intro / Body` regression from
CodeRabbit review

## Validation
- `pytest test/unit_test/deepdoc/parser/test_markdown_parser.py` (11
passed locally)

Closes #15487

test/unit_test/deepdoc/parser/test_markdown_parser.py

@@ -144,3 +144,20 @@ class TestMarkdownElementExtractorTables:
             "<table>\n<tr><td>A</td></tr>\n<tr><td>B</td></tr>\n</table>",
             "After",
         ]
+
+
+@pytest.mark.p2
+class TestMarkdownElementExtractorDelimiterHeaders:
+    def test_custom_delimiter_merges_consecutive_lone_headers_with_body(self, markdown_element_extractor):
+        text = "# Title\n## Intro\nBody paragraph"
+
+        sections = markdown_element_extractor(text).extract_elements(delimiter="`\n`")
+
+        assert sections == ["# Title\n## Intro\nBody paragraph"]
+
+    def test_custom_delimiter_merges_single_lone_header_with_body(self, markdown_element_extractor):
+        text = "## Section\nBody paragraph"
+
+        sections = markdown_element_extractor(text).extract_elements(delimiter="`\n`")
+
+        assert sections == ["## Section\nBody paragraph"]