feat: add java.sql.Clob converter

[AhmedYoussef98]

Purpose of the pull request

Related to #1017

What's changed?

Adds a default java.sql.Clob converter. Clob values are written to Excel as string cells by reading their character stream. String cells are converted back to detached SerialClob instances, avoiding database-specific implementations.

The converter is registered for default reads and writes. Unit tests cover converter keys, both conversion directions, and loader registration.

Verification

• ./mvnw package -B -Dmaven.test.skip=false -Dtest=ClobStringConverterTest,DefaultConverterLoaderTest -Dsurefire.failIfNoSpecifiedTests=false -pl fesod-common,fesod-shaded,fesod-sheet — 8 tests passed.
• ./mvnw spotless:check -B — passed.

The full spreadsheet suite did not complete on macOS because CompatibilityTest.readWithCacheAfterTempDeletion recursed repeatedly through FileUtils.delete; the targeted converter and loader tests passed.

Checklist

• I have read the Contributor Guide.
• I have added the necessary unit tests.

[skytin1004]

Thanks for adding this. I verified that the default converter handles a SerialClob correctly in the standard XLSX, XLS, and CSV read/write paths, and that the character stream is properly closed. All tests passed locally on Java 11. LGTM.

[nkuprins]

Is there a test gap on the write path? convertsClobCharacterStreamToCellString uses a 13-char value, so the 4096-char read loop only runs once. Changing while to if (so only the first chunk is kept) still passes all three tests. Using the cell text limit as the length covers several chunks and delei's boundary at the same time:

@Test
void convertsClobAtCellTextLimitToCellString() throws Exception {
    StringBuilder expected = new StringBuilder();
    for (int i = 0; i < 32767; i++) {
        expected.append((char) ('a' + i % 26));
    }
    Clob value = new SerialClob(expected.toString().toCharArray());

    WriteCellData<?> written = converter.convertToExcelData(value, null, GLOBAL_CONFIGURATION);

    Assertions.assertEquals(expected.toString(), written.getStringValue());
}

[AhmedYoussef98]

Added convertsClobAtCellTextLimitToCellString testing through 32,767 characters to verify chunking across buffer boundaries up to the Excel cell text limit. Thanks!

[Mikkey-f]

Hi @AhmedYoussef98 — checking in on this one. It has been quiet since late September, and the branch currently has conflicts with main.

Are you still planning to carry it forward? If you don't have the bandwidth for it right now, I'd be happy to pick it up — I'd rebase the converter on current main, keep the implementation and tests, adjust to the current testkit conventions where needed, and reference this PR and #1017 in the description so your work stays credited. Either way, just let me know.

[AhmedYoussef98]

Hi @Mikkey-f, thanks for checking in! I've just rebased onto main, resolved the conflicts in DefaultConverterLoader, addressed @nkuprins's comment with a 32k text-limit boundary test, and added a testkit parameterized round-trip test (ClobFieldRoundTripTest) across XLSX, XLS, and CSV. Ready for another look!

[Mikkey-f]

Great to hear — thanks for the quick turnaround on the rebase. I'll leave it with you then; withdrawing my offer.