fix: resolve conflict where global Date writer overrides @JSONField f…

[jujn]

…ormat, for issue #3864

What this PR does / why we need it?

Summary of your change

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[Copilot]

Pull request overview

This PR fixes a conflict where a globally registered Date writer was incorrectly overriding the @JSONField format annotation on Date fields. The fix ensures that when a Date field has a non-empty format specified via @JSONField, the field-specific format takes precedence over the global writer.

• Modified the condition in ObjectWriterCreator.createFieldWriter() to skip fetching the global writer when a Date field has a format annotation
• Added a comprehensive test case that verifies both scenarios: Date fields without format use the global writer, while Date fields with format use the specified format

Reviewed changes

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

File	Description
core/src/test/java/com/alibaba/fastjson2/issues_3800/Issue3864.java	Added test case demonstrating the fix: registers a global Date writer that outputs timestamps, then verifies that a Date field with @JSONField(format=...) correctly uses its format instead of the global writer
core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterCreator.java	Modified conditional logic to prevent global Date writer lookup when a Date field has a non-empty format annotation, allowing the format-specific writer to be used instead

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