test: cover field-level converter precedence

[skytin1004]

Purpose of the pull request

I've added test coverage for converter precedence when both a field-level converter and a registered converter could apply to the same Java type.

The behavior I wanted to lock down is that a converter declared directly on a field with @ExcelProperty(converter = ...) should take precedence over a converter registered through registerConverter(...).

What's changed?

Added a regression test in CustomConverterTest for this case.

The test writes a CSV with two String fields. One field has a field-level converter declared with @ExcelProperty, and the other field has no field-level converter, so it should use the registered String converter.

The expected output contains both converted values: field:value,registered:value.

This verifies the registered converter is actually active for the unannotated field and the field-level converter still takes precedence for the annotated field.

This is useful because users may register a general converter as a default behavior, while still expecting field-specific converters to override it for special cases.

Tested with fesod-sheet -Dmaven.test.skip=false -Dtest=CustomConverterTest test.

Result: Tests run: 7, Failures: 0, Errors: 0, Skipped: 0.

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]

Just noticed that #901 also changes CustomConverterTest.java. If #901 is merged first, I will rebase this PR and move this test to the new structure.

[psxjoy]

lgtm