fix: generate page_break for skipped pages in export functions

When pages fail to parse and are missing from the document,
the serializer now generates page_break markers for each
skipped page number instead of only for the next available page.

This ensures users can detect when pages were skipped during
PDF parsing by checking for non-consecutive page breaks.

Affects: export_to_doctags(), export_to_html(), export_to_markdown()

Author: jhchoi1182

docling_core/transforms/serializer/common.py

@@ -78,6 +78,36 @@ class _PageBreakSerResult(SerializationResult):
     node: _PageBreakNode
 
 
+def _yield_page_breaks(
+    prev_page: int,
+    next_page: int,
+    lvl: int,
+    start_index: int,
+) -> Iterable[Tuple[_PageBreakNode, int, int]]:
+    """Yield page break nodes for each page in range (prev_page, next_page].
+
+    When pages are non-consecutive (e.g., page 78 -> 82), this generates
+    individual page breaks for each transition (79, 80, 81, 82).
+
+    Args:
+        prev_page: The last seen page number.
+        next_page: The current page number.
+        lvl: The nesting level for the yielded nodes.
+        start_index: The starting index for page break node IDs.
+
+    Yields:
+        Tuples of (PageBreakNode, level, next_index) for each page transition.
+    """
+    idx = start_index
+    for page in range(prev_page + 1, next_page + 1):
+        yield _PageBreakNode(
+            self_ref=f"#/pb/{idx}",
+            prev_page=page - 1,
+            next_page=page,
+        ), lvl, idx + 1
+        idx += 1
+
+
 def _iterate_items(
     doc: DoclingDocument,
     layers: Optional[set[ContentLayer]],
@@ -113,22 +143,19 @@ def _iterate_items(
                     if isinstance(it, DocItem) and it.prov:
                         page_no = it.prov[0].page_no
                         if prev_page_nr is not None and page_no > prev_page_nr:
-                            yield _PageBreakNode(
-                                self_ref=f"#/pb/{page_break_i}",
-                                prev_page=prev_page_nr,
-                                next_page=page_no,
-                            ), lvl
+                            for pb_node, pb_lvl, page_break_i in _yield_page_breaks(
+                                prev_page_nr, page_no, lvl, page_break_i
+                            ):
+                                yield pb_node, pb_lvl
                         break
             elif isinstance(item, DocItem) and item.prov:
                 page_no = item.prov[0].page_no
                 if prev_page_nr is None or page_no > prev_page_nr:
                     if prev_page_nr is not None:  # close previous range
-                        yield _PageBreakNode(
-                            self_ref=f"#/pb/{page_break_i}",
-                            prev_page=prev_page_nr,
-                            next_page=page_no,
-                        ), lvl
-                        page_break_i += 1
+                        for pb_node, pb_lvl, page_break_i in _yield_page_breaks(
+                            prev_page_nr, page_no, lvl, page_break_i
+                        ):
+                            yield pb_node, pb_lvl
                     prev_page_nr = page_no
         yield item, lvl