fix: optimize Excel row counting for files with abnormal max_row (#13018)

### What problem does this PR solve?

Some Excel files have abnormal `max_row` metadata (e.g.,
`max_row=1,048,534` with only 300 actual data rows). This causes:
- `row_number()` returns incorrect count, creating 350+ tasks instead of
1
- `list(ws.rows)` iterates through millions of empty rows, causing
system hang

This PR uses binary search to find the actual last row with data.

### Type of change

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

Co-authored-by: Cursor <cursoragent@cursor.com>

rag/app/table.py

@@ -44,7 +44,7 @@ class Excel(ExcelParser):
             wb = Excel._load_excel_to_workbook(BytesIO(binary))
         total = 0
         for sheet_name in wb.sheetnames:
-            total += len(list(wb[sheet_name].rows))
+            total += Excel._get_actual_row_count(wb[sheet_name])
         res, fails, done = [], [], 0
         rn = 0
         flow_images = []
@@ -66,7 +66,7 @@ class Excel(ExcelParser):
                             flow_images.append(img)
 
             try:
-                rows = list(ws.rows)
+                rows = Excel._get_rows_limited(ws)
             except Exception as e:
                 logging.warning(f"Skip sheet '{sheet_name}' due to rows access error: {e}")
                 continue