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.