refactor: extract format-string date detection out of DateUtils

[nkuprins]

Closed: #1062

Purpose of the pull request

As title

What's changed?

• Extracted date format detection: moved detection logic out of DateUtils into a dedicated ExcelDateFormatDetector package-private class.
• Updated test references: adjusted DateUtilsTest to test date format detection via the newly extracted ExcelDateFormatDetector.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[skytin1004]

I reviewed this locally and ran the relevant tests, which all passed.

I confirmed that the date detection logic was moved without changes and that the existing public methods still work through DateUtils. The cache cleanup test also confirms that DateUtils.removeThreadLocalCache() clears the cache in the new class.

Since this refactoring moves the date detection logic without changing its behavior, I think the existing tests provide sufficient coverage. LGTM.