fix: clone DataFormatData in WriteCellStyle.merge to avoid mutating cached annotation style

[nkuprins]

Purpose of the pull request

Closed: #956

What's changed?

WriteCellStyle.merge copied the source's DataFormatData into the target by reference. Because the annotation-driven style source is parsed once and cached, a later merge from another style source (e.g. a registered HorizontalCellStyleStrategy) wrote into that shared object and permanently changed the cached @ContentStyle(dataFormat) - producing the wrong number format on rows where the strategy defines no format, and (with CacheLocationEnum.MEMORY) on later, unrelated writes of the same bean class.

The fix clones the DataFormatData before assigning it, exactly like writeFont is already cloned a few lines below in the same method:

- target.setDataFormatData(source.getDataFormatData());
+ target.setDataFormatData(source.getDataFormatData().clone());

Added ContentStyleDataFormatPollutionTest with two regression tests that assert on the number format of the written file (via the testkit's ExcelAssertions):

• annotationFormatSurvivesStyleStrategyInSameWrite - default config: a row whose strategy style has no data format must keep the annotation's #,##0.00.
• annotationFormatSurvivesAcrossWritesWithMemoryCache - MEMORY cache location: a follow-up write with no custom handlers must keep the annotation's format.

Both tests fail on main (format comes out 0.00) and pass with the fix. The full fesod-sheet suite passes with the fix applied (673 tests, 0 failures, 0 errors, 0 skipped), confirming no behavior change for styles that don't involve a shared DataFormatData.

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.

[Copilot]

Pull request overview

This pull request fixes a subtle style-caching mutation bug in fesod-sheet where WriteCellStyle.merge previously copied DataFormatData by reference, allowing later merges (e.g., from a HorizontalCellStyleStrategy) to corrupt the cached annotation-driven @ContentStyle(dataFormat) metadata and produce incorrect number formats.

Changes:

• Clone DataFormatData when assigning it during WriteCellStyle.merge to prevent shared-reference mutation of cached annotation style metadata.
• Add ContentStyleDataFormatPollutionTest regression coverage to ensure the annotation format survives (1) within the same write when a strategy provides no format, and (2) across writes when CacheLocationEnum.MEMORY is used.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
fesod-sheet/src/main/java/org/apache/fesod/sheet/write/metadata/style/WriteCellStyle.java	Prevents cached annotation DataFormatData from being aliased/mutated by cloning on merge assignment.
fesod-sheet/src/test/java/org/apache/fesod/sheet/style/ContentStyleDataFormatPollutionTest.java	Adds regression tests that assert on the written file’s number formats to prevent recurrence (same-write + MEMORY-cache scenarios).

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