test: add testkit infrastructure and refactor unit tests with JUnit 5 parameterized patterns

Implements Phase 1-2 of the test modernization proposal from issue #717:

- Add testkit/ package with AbstractExcelTest, ExcelFormat enum,
  CollectingReadListener, and ExcelAssertions for shared test infrastructure
- Migrate converter and other legacy test packages to parameterized JUnit 5 tests
- Remove listener classes with embedded assertions (SRP violation)
- Consolidate scattered model classes into organized packages (core/, format/,
  readwrite/, sheet/, style/)
- Apply parameterized test pattern to cover XLSX/XLS/CSV in single methods
- Update DateUtils to support test infrastructure

Closes #717

Author: GOODBOY008

fesod-sheet/src/main/java/org/apache/fesod/sheet/util/DateUtils.java

@@ -549,9 +549,9 @@ public static boolean isADateFormatUncached(Short formatIndex, String formatStri
      */
     public static boolean isInternalDateFormat(short format) {
         switch (format) {
-            // Internal Date Formats as described on page 427 in
-            // Microsoft Excel Dev's Kit...
-            // 14-22
+                // Internal Date Formats as described on page 427 in
+                // Microsoft Excel Dev's Kit...
+                // 14-22
             case 0x0e:
             case 0x0f:
             case 0x10:
@@ -561,7 +561,7 @@ public static boolean isInternalDateFormat(short format) {
             case 0x14:
             case 0x15:
             case 0x16:
-            // 45-47
+                // 45-47
             case 0x2d:
             case 0x2e:
             case 0x2f:

fesod-sheet/src/test/java/org/apache/fesod/sheet/FesodSheetTest.java

@@ -189,7 +189,7 @@ void testRead_noArgs_shouldReturnBuilder() {
     }
 
     @Test
-    void testRead_withFile_shouldConfigureFile() throws Exception {
+    void testRead_withFile_shouldConfigureFile() {
         ExcelReaderBuilder builder = FesodSheet.read(tempFile);
         assertNotNull(builder);
         ReadWorkbook workbook = writeWorkbook(builder);