FileUploader: Implement internal file count limitation

[marker-dao]

No description provided.

[Copilot]

Pull Request Overview

This PR implements an internal file count limitation feature for the FileUploader component. The feature allows developers to set a maximum number of files that can be uploaded and provides a callback when this limit is reached.

Key Changes:

• Added _maxFileCount option to limit the number of files that can be selected
• Implemented onFileLimitReached event callback that fires when the file limit is exceeded
• Added file count validation for both file input selection and drag-and-drop operations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/devextreme/js/__internal/ui/file_uploader/file_uploader.types.ts	Added type definitions for FileLimitReachedEvent and the _maxFileCount and onFileLimitReached options
packages/devextreme/js/__internal/ui/file_uploader/file_uploader.ts	Implemented file limit validation logic, event handling, and integration with file input and drag-drop workflows
packages/devextreme/testing/tests/DevExpress.ui.widgets.editors/fileUploader.tests.js	Added comprehensive test coverage for the file limit feature across multiple scenarios

[Copilot]

The file limit check in _createFiles() incorrectly checks if the current files already exceed the limit, but doesn't prevent the files from being processed. This check should occur before files are added to the component, not after. The check here will always pass the current value to _isFileLimitReached, which will compare files.length + files.length, effectively doubling the count. This logic should be removed as the limit is already enforced in _inputChangeHandler and _dropHandler.

Suggested change

	if (this._isFileLimitReached(files as unknown as File[])) {
	this._fileLimitReachedAction?.();
	}
	// File limit is enforced in _inputChangeHandler and _dropHandler

[marker-dao]

This logic is necessary when programmatically changing the value option or during the first render. But we can remove it after discussion with the team.