Test pull diagnostics in JSON, HTML and CSS

[aeschli]

Refs: #149643

• AnyOS @tanhakabir CSS
• AnyOS @bamurtaugh JSON
• AnyOS @dbaeumer HTML

Complexity: 2

Create Issue

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The implementation of the CSS/HTML/JSON language servers has changed in how diagnostics are processed. They are now requested by VS Code instead of pushed by the language server. Nothing should change for the user.
This issue is to verify that all behaves normal

• open a CSS/HTML/JSON file
• create some error, you should see error squiggles being shown and an error shown in the problem view
• fix the error, the squiggles are going away.
• Note: HTML validates only happens in <script> and `<style> blocks, not for HTML tags or attributes
• verify that only errors are shown on the problems view for editors that are open. Closing an editor should hide the errors
• verify with errors diffs, in read-only editors such as git history.