perf: optimize ExcelIO tests with fixture-based consolidation

- Add WorkbookFixtureBase and 4 concrete fixtures for shared workbooks
- Add ExcelTestHelpers with shared utility methods
- Create Integration/ folder with 7 fixture-based test classes:
  - MinimalSearchTests (5 tests)
  - SearchResultsSheetTests (5 tests)
  - MisInfoSheetTests (11 tests)
  - InvestigationSheetTests (7 tests)
  - ProtectionAndStyleTests (7 tests)
  - LegacyFormatTests (5 tests)
  - LargeDataSetTests (1 test)
- Delete redundant ExcelExportIntegrationTests.cs (26 tests)
- Delete redundant LegacyComparisonTests.cs (16 tests)
- Reduce workbook generations from ~42 to 4 fixtures
- Test runtime reduced from ~18 min to ~4 min (76% improvement)
- All 122 ExcelIO tests pass

NEW/tests/JdeScoping.ExcelIO.Tests/Fixtures/ExcelTestHelpers.cs

@@ -0,0 +1,18 @@
+using ClosedXML.Excel;
+
+namespace JdeScoping.ExcelIO.Tests.Fixtures;
+
+public static class ExcelTestHelpers
+{
+    public static List<string> GetHeadersFromSheet(IXLWorksheet sheet)
+    {
+        var headers = new List<string>();
+        var col = 1;
+        while (!sheet.Cell(1, col).IsEmpty())
+        {
+            headers.Add(sheet.Cell(1, col).Value.GetText());
+            col++;
+        }
+        return headers;
+    }
+}