Add video tutorial link to the Deep Links panel

[binSaed]

Description

Most DevTools panels display a "Watch tutorial" link next to the docs link in the status line. The Deep Links panel is missing this link because it doesn't have a tutorialVideoTimestamp in its ScreenMetaData.

The Deep Links panel has its own dedicated video tutorial (https://www.youtube.com/watch?v=d7sZL6h1Elw) which is separate from the main "Dive in to DevTools" video used by other panels.

This PR adds a tutorialVideoUrl field to ScreenMetaData that allows a screen to specify a custom video URL. When set, VideoTutorialLink uses this URL instead of the default DevTools YouTube video. The "Watch tutorial" link now appears on the Deep Links panel.

Changes

• packages/devtools_app/lib/src/shared/framework/screen.dart: Added tutorialVideoUrl field to ScreenMetaData and set it for the deepLinks screen.
• packages/devtools_app/lib/src/framework/scaffold/status_line.dart: Updated showVideoTutorial check and VideoTutorialLink to use tutorialVideoUrl when available.

Fixes #9425

[gemini-code-assist]

Code Review

This pull request introduces support for custom video tutorial URLs (tutorialVideoUrl) in ScreenMetaData, allowing individual screens to link to dedicated video tutorials instead of falling back to a timestamp on the default DevTools YouTube video. Feedback on the changes highlights two main areas of improvement: handling cases where both tutorialVideoUrl and tutorialVideoTimestamp are null to prevent generating an invalid URL containing the string 'null', and using the shortened youtu.be domain format for the new URL to maintain consistency with other YouTube links in the codebase.

[kenzieschmoll]

Thanks for the contribution. Overall looks good. Just a couple of comments.

[binSaed]

Thanks for the review! Both comments addressed — PTAL.