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Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Previously, only MaxFileSizeValidator supported custom messages, while FileTypeValidator did not, which caused confusion and made it difficult to deliver service-appropriate messages.
Errors were also returned as simple strings, requiring unnecessary branching through the exceptionFactory.
Now, the usage of MaxFileSizeValidator and FileTypeValidator—specifically the way custom messages are delivered—has been standardized.

• close: Customizing FileType Validator Error Messages #16058

What is the new behavior?

By enabling the use of custom messages in FileTypeValidator—as was already possible in MaxFileSizeValidator—it is now possible to deliver service-appropriate messages and maintain consistency in how MaxFileSizeValidator and FileTypeValidator are used.
Without using exceptionFactory, simple strings can now be used in FileTypeValidator to provide messages for Bad Request responses.

In addition, the existing message option has been deprecated in favor of a new
errorMessage option for clearer semantics and consistency across validators.

The deprecated message option is still supported for backward compatibility,
but errorMessage is now the preferred way to provide custom error messages.

new MaxFileSizeValidator({ maxSize: 1000, message: 'Error'}); // 🟢 Although deprecated, this option is still supported.
new MaxFileSizeValidator({ maxSize: 1000, errorMessage: 'Error'}); // 🟢 New Property
new MaxFileSizeValidator({ maxSize: 1000, errorMessage: ctx =>
          `Received file size is ${ctx.file?.size}, but it must be smaller than ${ctx.config.maxSize}.`
}); // 🟢 New Property with factory function

new FileTypeValidator({ fileType: 'image/png', message: 'invalid type'}); // ❌ Since this property did not previously exist, it cannot be used.
new FileTypeValidator({ fileType: 'image/png', errorMessage: 'invalid type'}); // 🟢New Property
new FileTypeValidator({ fileType: 'image/png', errorMessage: ctx =>
          `Received file type '${ctx.file?.mimetype}', but expected '${ctx.config.fileType}'.`
});  // 🟢 New Property with factory function

Does this PR introduce a breaking change?

• Yes
• No

Other information