Pull request overview

This pull request fixes a converter isolation bug where custom converters registered in one ExcelWriter or ExcelReader instance would leak to other instances due to shared mutable static maps.

Key Changes:

• Wrapped default converter maps with Collections.unmodifiableMap() in DefaultConverterLoader to prevent direct modification
• Modified holder initialization to create defensive mutable copies of converter maps for each instance
• Added test case to verify converter isolation between ExcelWriter instances

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
fesod/src/main/java/org/apache/fesod/sheet/converters/DefaultConverterLoader.java	Returns unmodifiable maps from loadDefaultWriteConverter() and loadAllConverter() methods to protect static converter maps
fesod/src/main/java/org/apache/fesod/sheet/write/metadata/holder/AbstractWriteHolder.java	Creates mutable HashMap copy from unmodifiable default converter map during holder initialization
fesod/src/main/java/org/apache/fesod/sheet/read/metadata/holder/AbstractReadHolder.java	Creates mutable HashMap copy from unmodifiable default converter map during holder initialization
fesod/src/test/java/org/apache/fesod/sheet/converter/ConverterIsolationTest.java	Adds new test to validate custom converters do not leak between ExcelWriter instances

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