fix(docx): handle tables with merged cells causing IndexError (#2813)

* fix(docx): handle tables with merged cells causing IndexError

When parsing tables with merged cells, the number of cells in a row
can be less than the total number of columns. The code was iterating
up to num_cols and accessing row.cells[col_idx], which caused
IndexError when col_idx exceeded the actual number of cells in the row.

Added bounds check before accessing row.cells to handle merged cell rows.

* DCO Remediation Commit for Maxym Tushkov <maxym.nc@gmail.com>

I, Maxym Tushkov <maxym.nc@gmail.com>, hereby add my Signed-off-by to this commit: 144047a

Signed-off-by: Maxym Tushkov <maxym.nc@gmail.com>

---------

Signed-off-by: Maxym Tushkov <maxym.nc@gmail.com>
Co-authored-by: Maxym Tushkov <maxym.nc@gmail.com>

Author: madmaxxt

docling/backend/msword_backend.py

@@ -1351,6 +1351,9 @@ def _handle_tables(
             _log.debug(f"Row index {row_idx} with {len(row.cells)} populated cells")
             col_idx = 0
             while col_idx < num_cols:
+                # Handle merged cells: row may have fewer cells than num_cols
+                if col_idx >= len(row.cells):
+                    break
                 cell: _Cell = row.cells[col_idx]
                 _log.debug(
                     f" col {col_idx} grid_span {cell.grid_span} grid_cols_before {row.grid_cols_before}"