fix: Prevent starting the same file download twice

[Ivansss]

Fixes #2694

• The fileParameter.fileStatus?.isDownloading guards in the chat could never be true since feat: Improve file downloading handling #2598, so every tap started another download of the same file. Chat downloads now go through ChatFileDownloader, which already shared one download between callers asking for the same file.
• NCChatFileController reports through a completion handler instead of a delegate, so the caller's intent stays at the call site. This removes messageType and actionType, which only existed to tell the callbacks apart again.
• The completion handler is now called exactly once on every path, including cancellation. Before, cancelling dropped the handler, which would have left ChatFileDownloader with an entry it never cleans up, blocking that file for the rest of the session.
• Renamed NCChatFileControllerWrapper to ChatFileDownloader: it no longer wraps a delegate, and the name now matches ChatFileUploader.
• Removed the dead fileStatus branch in BaseChatTableViewCell+File.

Not changed here: the media viewer keeps its own NCChatFileController because it needs to cancel downloads for pages the user swiped past, and ChatFileDownloader deliberately has no cancellation. Repeated requests for the same file still queue one handler each, as before.

How to test

1. Tap a PDF in a conversation repeatedly while it downloads -> only one download runs.
2. Play a voice message, transcribe one, open an image and a video, open a file from Shared items, open the file of a file conversation -> all still work.
3. Swipe quickly through several images in the media viewer -> downloads of skipped pages are still cancelled.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI