Fix/external upload limit

[Kota-Jagadeesh]

This version is enhanced from the initial proposal (#6555) and includes key refactorings:

1. user-Friendly truncation (instead of Hard Block):

• In UploadActivity.kt, the upload list is now checked for size after file processing.
• If fileCount > 20, the list is immediately truncated to the MAX_EXTERNAL_UPLOAD_COUNT.

A non-blocking toast is shown to the user informing them that only the first 20 files were loaded, ensuring workflow continuity with clear feedback.

2. Constant Centralization (Refactoring):

• The hardcoded limit (20) was removed from CustomSelectorActivity.kt and centralized into a dedicated shared constant: fr.free.nrw.commons.filepicker.Constants.MAX_EXTERNAL_UPLOAD_COUNT.

Both UploadActivity.kt and CustomSelectorActivity.kt now correctly reference this centralized constant, improving code maintainability and adherence to DRY principles.

3. New String Resource:

• Added R.string.multiple_upload_limit_truncated_message to inform the user about the truncation.

[github-actions]

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