[Bee] Excel backend duplicates cells of a sparse table as extra tables

### Bug

When a worksheet has a single table with empty cells, non-empty cells that are not directly adjacent to the rest of the table (e.g. a column whose first value has an empty cell above it) are exported twice: once inside the main table, and again as a separate small table.

Minimal sheet (`sparse_sheet.xlsx`): [sparse_sheet.xlsx](https://github.com/user-attachments/files/32133418/sparse_sheet.xlsx)

Expected: 1 table of shape (4, 3).
Actual: 2 tables, shapes `[(4, 3), (2, 1)]`. Markdown export:

```
| ID | Name | Note |
|------|--------|--------|
| 1 | alpha | |
| 2 | | foo |
| 3 | | bar |

| foo |
|-------|
| bar |
```

`foo`/`bar` already appear in the first table and are repeated as a standalone table. On real sparse sheets (e.g. ~100-column exports) this produces many such duplicate fragment tables.

### Steps to reproduce

Attached: `sparse_sheet.xlsx` and the code:

```python
from pathlib import Path
from docling.document_converter import DocumentConverter

xlsx = Path("sparse_sheet.xlsx")
doc = DocumentConverter().convert(xlsx).document
xlsx.with_suffix(".md").write_text(doc.export_to_markdown())

shapes = [(t.data.num_rows, t.data.num_cols) for t in doc.tables]
print(f"actual: {len(doc.tables)} tables of shapes {shapes}")
assert shapes == [(4, 3)]
```

Output:

```
actual: 2 tables of shapes [(4, 3), (2, 1)]
AssertionError
```

### Docling version

```
Docling version: 2.110.0
Docling Core version: 2.91.0
Docling IBM Models version: 3.14.0
```

### Python version

```
Python 3.12.12
```

Start with the attached sparse_sheet.xlsx and the DocumentConverter reproduction shown in the issue, then trace the Bee Excel backend while inspecting how doc.tables are assembled. Confirm the current result has shapes [(4, 3), (2, 1)]; done means the sparse sheet produces one table of shape (4, 3) and no duplicated fragment in Markdown export.

Written by the indexing model from the issue text.