Video Block: Guard against duplicate tracks

[yogeshbhutkar]

What?

Closes #70294

This PR updates the media upload's onSelect handler to return early if the selected source already exists in the track list, and notifies the user about the duplicate entry.

Why?

Since the source is used as the key when mapping tracks, duplicate entries trigger a React warning about non-unique keys.

How?

The onSelect handler prevents further processing if the selected source is already present.

Testing Instructions

1. Create a new post.
2. Insert a Video block.
3. Try attaching duplicate tracks to the video block
4. Confirm that it's not possible to attach duplicate tracks to the video block.

Testing Instructions for Keyboard

Same.

Screencast

PR.mov

[yogeshbhutkar]

Using the file's title as the label makes more sense than falling back to an arbitrary default (such as 'English' in this case) when the label is empty. Since this is a minor change, I've included it as part of this small refactor.

P.S. I plan to refactor the use of defaults in tracks and introduce internal state management for the single track UI in a follow-up.

Ref. #70227 (comment)

[github-actions]

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

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[Mamaduka]

@yogeshbhutkar, do you mind rebasing this PR on top of the latest trunk?

[Mamaduka]

Works as expected. Thank you, @yogeshbhutkar!