fix: scope pending_cell_images by sheet in excel parser (#14120)

pending_cell_images should be scoped by sheet

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

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

rag/app/table.py

@@ -50,11 +50,11 @@ class Excel(ExcelParser):
         res, fails, done = [], [], 0
         rn = 0
         flow_images = []
-        pending_cell_images = []
         tables = []
         for sheet_name in wb.sheetnames:
             ws = wb[sheet_name]
             images = Excel._extract_images_from_worksheet(ws, sheetname=sheet_name)
+            pending_cell_images = []
             if images:
                 image_descriptions = vision_figure_parser_figure_xlsx_wrapper(images=images, callback=callback,
                                                                               **kwargs)