fix: avoid recursive hashCode in write holder contexts

[skytin1004]

Purpose of the pull request

Related: #889

This PR addresses only the recursive hashCode() failure path discussed in #889 and #899.

While rechecking the original failure, I found that the custom converter reproduction appears to involve at least two separate concerns:

1. recursive hashCode() evaluation in write holder/context objects
2. global write converter lookup when a registered converter declares a non-null supportExcelTypeKey()

To keep the review focused, this PR deliberately handles only the first issue.

I reproduced the recursive hashCode() paths locally and added a focused regression test for them:
The regression test creates the recursive holder/context references directly and then calls hashCode(), so it covers this PR's failure path without depending on converter behavior.

• WriteWorkbookHolder.hashCode() -> WorkbookWriteHandlerContext.hashCode() -> WriteWorkbookHolder.hashCode()
• WriteWorkbookHolder.hashCode() -> initialized sheet holder map -> WriteSheetHolder.hashCode() -> parent workbook holder -> WriteWorkbookHolder.hashCode()

The fix excludes only the recursive runtime references from Lombok-generated equals/hashCode:

• WriteWorkbookHolder.workbookWriteHandlerContext
• WriteSheetHolder.parentWriteWorkbookHolder
• WriteSheetHolder.hasBeenInitializedTable
• WriteTableHolder.parentWriteSheetHolder

This PR intentionally does not change converter lookup behavior. I plan to handle that separately with a smaller follow-up test/fix, because it can fail independently when a global converter is registered with a non-null supportExcelTypeKey().

I verified the change locally by running:

• the new WriteHolderHashCodeTest
• the existing CustomConverterTest
• the full fesod-sheet test suite

All tests passed.

Note

If this recursive hashCode() fix looks good and gets merged, I can follow up with a smaller PR focused only on the global write converter lookup case, so that the remaining part of #889 can be addressed clearly.

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.