Fix: Excel parser emits a spurious header-only chunk at exact chunk_rows multiples (#15490)

### What problem does this PR solve?

`RAGFlowExcelParser.html()` iterates `(len(rows) - 1) // chunk_rows + 1`
times. `rows[0]` is the header, so `len(rows) - 1` is the data-row
count. When that count is an exact multiple of `chunk_rows`, the `+ 1`
over-counts by one: the final iteration's data slice is empty, but the
header row is still appended — producing a chunk that contains only the
table header and no data.

This is reachable via `rag/app/naive.py` (`html4excel`, `chunk_rows=12`)
and `rag/app/one.py`. A sheet with 12/24/36… data rows (or 256/512… with
the default `chunk_rows=256`) produces an extra
`<table><caption>…</caption><tr><th>…</th></tr></table>` chunk. It is
non-empty, so it passes the `if _` filter and gets indexed as a real
(empty) chunk.

| data rows (chunk_rows=12) | before | after |
|---|---|---|
| 12 | 2 chunks (1 header-only) | 1 |
| 24 | 3 chunks (1 header-only) | 2 |
| 13 | 2 (unchanged) | 2 |

### Fix

Iterate `ceil(n_data / chunk_rows)` times instead of `n_data //
chunk_rows + 1`. Adds
`test/unit_test/deepdoc/parser/test_excel_parser.py`; the
header-only-chunk cases fail before this change and pass after.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Used the Claude CLI while working on this.

test/unit_test/deepdoc/parser/test_excel_parser.py

@@ -0,0 +1,92 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+import importlib.util
+import os
+import sys
+from io import BytesIO
+from unittest import mock
+
+import pytest
+
+# Import RAGFlowExcelParser directly by file path to avoid triggering
+# deepdoc/parser/__init__.py and rag.nlp, which pull in heavy dependencies.
+for _m in ["pandas", "rag.nlp", "rag.utils", "rag.utils.lazy_image"]:
+    if _m not in sys.modules:
+        sys.modules[_m] = mock.MagicMock()
+
+
+def _find_project_root(marker="pyproject.toml"):
+    d = os.path.dirname(os.path.abspath(__file__))
+    while d != os.path.dirname(d):
+        if os.path.exists(os.path.join(d, marker)):
+            return d
+        d = os.path.dirname(d)
+    return None
+
+
+_PROJECT_ROOT = _find_project_root()
+_spec = importlib.util.spec_from_file_location(
+    "deepdoc.parser.excel_parser",
+    os.path.join(_PROJECT_ROOT, "deepdoc", "parser", "excel_parser.py"),
+)
+_mod = importlib.util.module_from_spec(_spec)
+sys.modules["deepdoc.parser.excel_parser"] = _mod
+_spec.loader.exec_module(_mod)
+
+RAGFlowExcelParser = _mod.RAGFlowExcelParser
+
+
+def _make_xlsx(n_data_rows):
+    from openpyxl import Workbook
+
+    wb = Workbook()
+    ws = wb.active
+    ws.append(["H1", "H2"])
+    for i in range(n_data_rows):
+        ws.append([f"a{i}", f"b{i}"])
+    buf = BytesIO()
+    wb.save(buf)
+    buf.seek(0)
+    return buf.read()
+
+
+def _chunk_has_no_data_cells(chunk):
+    return "<td>" not in chunk and "<td></td>" not in chunk
+
+
+@pytest.mark.p2
+def test_exact_multiple_does_not_emit_header_only_chunk():
+    # 12 data rows with chunk_rows=12 (the value rag/app/naive.py uses).
+    chunks = RAGFlowExcelParser().html(_make_xlsx(12), chunk_rows=12)
+    assert len(chunks) == 1
+    assert all(not _chunk_has_no_data_cells(c) for c in chunks)
+
+
+@pytest.mark.p2
+def test_multiple_of_chunk_rows_splits_without_spurious_chunk():
+    # 24 data rows with chunk_rows=12 -> exactly 2 data chunks, no trailing header-only chunk.
+    chunks = RAGFlowExcelParser().html(_make_xlsx(24), chunk_rows=12)
+    assert len(chunks) == 2
+    assert all(not _chunk_has_no_data_cells(c) for c in chunks)
+
+
+@pytest.mark.p2
+def test_non_multiple_unchanged():
+    # 13 data rows with chunk_rows=12 -> 2 chunks (12 + 1).
+    chunks = RAGFlowExcelParser().html(_make_xlsx(13), chunk_rows=12)
+    assert len(chunks) == 2
+    assert all(not _chunk_has_no_data_cells(c) for c in chunks)