[Bug] csv和xlsx两种模式不能共用同一个自定义Converter

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fesod/issues) and found nothing similar.

### Fesod version

2.0.1-incubating

### JDK version

8

### Operating system

macOS 15.2

### Steps To Reproduce

单测如下：
```java
class FesodCsvConverterKeyNativeTest {

/** JVM 全局静态写转换器表的初始快照（本类首个测试运行前保存） */
private static Map pristineConverterMap;

@BeforeAll
static void snapshotGlobalConverterMap() throws Exception {
pristineConverterMap = new HashMap<>(globalWriteConverterMap());
}

@BeforeEach
void restoreGlobalConverterMap() throws Exception {
Field field = DefaultConverterLoader.class.getDeclaredField("defaultWriteConverter");
field.setAccessible(true);
// 用快照的拷贝整体替换，抹掉此前测试 registerConverter 留下的全局污染
field.set(null, new HashMap<>(pristineConverterMap));
}

@SuppressWarnings("unchecked")
private static Map globalWriteConverterMap() throws Exception {
Field field = DefaultConverterLoader.class.getDeclaredField("defaultWriteConverter");
field.setAccessible(true);
return (Map) field.get(null);
}

/** 内联自定义转换器：Boolean -> 是/否 */
static class BooleanToChineseConverter implements Converter {
@Override
public Class supportJavaTypeKey() {
return Boolean.class;
}

@Override
public CellDataTypeEnum supportExcelTypeKey() {
// 返回 null 表示匹配所有 Excel 类型 —— 注册 key 为 (Boolean, null)
return null;
}

@Override
public WriteCellData convertToExcelData(WriteConverterContext context) {
Boolean value = context.getValue();
if (Boolean.TRUE.equals(value)) {
return new WriteCellData<>("是");
}
if (Boolean.FALSE.equals(value)) {
return new WriteCellData<>("否");
}
return new WriteCellData<>("");
}
}

@Test
@DisplayName("xlsx 下自定义转换器正常生效：Boolean 输出「是」✅")
void xlsxShouldApplyCustomConverter(@TempDir Path tempDir) throws Exception {
File file = tempDir.resolve("custom.xlsx").toFile();

FesodSheet.write(file)
.excelType(ExcelTypeEnum.XLSX)
.registerConverter(new BooleanToChineseConverter())
.sheet()
.doWrite(Collections.singletonList(Collections.singletonList(Boolean.TRUE)));

try (Workbook workbook = WorkbookFactory.create(file)) {
String cellValue = workbook.getSheetAt(0).getRow(0).getCell(0).getStringCellValue();
System.out.println("xlsx + 自定义转换器: " + cellValue);
// targetCellDataType 未被改写（null），按 (Boolean, null) 命中自定义转换器
assertEquals("是", cellValue);
}
}

@Test
@DisplayName("CSV 下同一转换器被内置 String 转换器静默顶替：Boolean 输出 true 而非「是」（缺陷复现❌）")
void csvShouldSilentlyReplaceCustomConverter(@TempDir Path tempDir) throws Exception {
File file = tempDir.resolve("replaced.csv").toFile();

FesodSheet.write(file)
.excelType(ExcelTypeEnum.CSV)
.charset(StandardCharsets.UTF_8)
.registerConverter(new BooleanToChineseConverter())
.sheet()
.doWrite(Collections.singletonList(Collections.singletonList(Boolean.TRUE)));

String content = new String(Files.readAllBytes(file.toPath()), StandardCharsets.UTF_8);
System.out.println("CSV + 自定义转换器: " + content);
// 强制按 (Boolean, STRING) 查找，key (Boolean, null) 的自定义转换器未命中，
// 被内置 BooleanStringConverter 顶替，输出 value.toString() 即 true/false
assertEquals("\uFEFFtrue\r\n", content);
}

```

### Current Behavior

不能共用同一个Converter，csv模式下被内置 String 转换器静默顶替

### Expected Behavior

能共用同一个Converter

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Start with the FesodCsvConverterKeyNativeTest reproduction and inspect DefaultConverterLoader.defaultWriteConverter together with the converter registration and lookup paths used by XLSX and CSV. Run both tests to compare their converter keys; done means the same custom Boolean converter produces 「是」 in both formats and the regression tests pass.

Written by the indexing model from the issue text.