Add a progress indicator with cancel for Switch Header/Source.

[sean-mcmanus]

Fixes #14379 .

Implemented by Copilot (took a few iterations though).

Waits 2 seconds before showing the notification (same as Find All References).

[Copilot]

Pull request overview

Adds a cancellable progress notification for the Switch Header/Source command to address the lack of user feedback and inability to stop long-running switch operations (per issue #14379), with a 2-second delay before showing the notification (matching the Find All References UX).

Changes:

• Wrap onSwitchHeaderSource execution in a delayed withProgress notification that supports user cancellation.
• Plumb a vscode.CancellationToken through the extension → client request path for Switch Header/Source.
• Translate LSP cancellation response codes into vscode.CancellationError for consistent cancellation handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Extension/src/LanguageServer/extension.ts	Adds delayed progress notification + cancel wiring around Switch Header/Source and handles cancellation cleanly.
Extension/src/LanguageServer/client.ts	Updates requestSwitchHeaderSource to accept a cancellation token and maps LSP cancellation to vscode.CancellationError.

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