Skip blank rows and section headers in Excel processing

[Copilot]

Excel loader was breaking at first blank row, ignoring all questions below it. Section headers (e.g., "Section 2: Cloud Computing") also stopped processing.

Changes

• Skip blank rows: Continue iterating instead of breaking on empty cells
• Detect section headers: Heuristic-based detection for common patterns:
  • Section prefixes (Section 1:, Part A:, Chapter 3:)
  • Decorated text (--- Category ---, === Section ===)
  • All uppercase short text
  • Numbered sections without question marks
• Track row indices: Store original Excel row numbers in SheetData.row_indices so answers write back to correct positions

# Before: stops at row 4
# Row 2: Question 1  →  processed
# Row 3: Question 2  →  processed  
# Row 4: (blank)     →  BREAK
# Row 5: Question 3  →  never processed

# After: skips blanks and headers, tracks original positions
questions = ["Question 1", "Question 2", "Question 3"]
row_indices = [2, 3, 5]  # answers written to correct rows

Files

• src/excel/loader.py: Modified loading logic, added _is_section_header()
• src/utils/data_types.py: Added row_indices: Optional[List[int]] to SheetData
• tests/unit/test_excel_blank_rows.py: 9 new tests for header detection and blank row handling

Original prompt

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This section details on the original issue you should resolve

<issue_title>System should ignore blank rows in Excel sheets</issue_title>
<issue_description>If there's a blank row, or a question cell seems to contain something that isn't a question (like a section header) as judged up front by the large language model, then the system should just ignore that row and move on. Right now it stops right at the blank or header row and does not proceed with questions below it.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes System should ignore blank rows in Excel sheets #72

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