refined waitUntil with document URI to fire as soon as document reaches required state

[sailingKieler]

@msujew here's my proposal on a refinement of DocumentBuilder.waitUntil(...) as discussed.

Is the way of reporting wrong pre-conditions via rejecting with ResponseErrors the way to go?
I enforced them with VS Code but didn't see any outputs in the log or did notice any UI pop-ups...

[Copilot]

Pull Request Overview

Refines the waitUntil method in DocumentBuilder to support waiting for specific documents to reach required states, with improved error handling and immediate resolution when documents are already in the required state.

• Separates document-specific waiting logic from general builder state waiting
• Adds comprehensive error handling for missing documents and invalid state transitions
• Includes a test case to verify the new behavior fires immediately when a document reaches the required state

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/langium/src/workspace/document-builder.ts	Splits waitUntil into separate methods for document and builder state waiting, adds error handling with ResponseError
packages/langium/test/workspace/document-builder.test.ts	Adds test case to verify immediate firing when document reaches required state
packages/langium/src/lsp/language-server.ts	Ensures workspace manager is ready before waiting for document states

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[Copilot]

Using LSPErrorCodes.ServerCancelled for a missing document is semantically incorrect. This error code indicates the server cancelled the request, not that a document wasn't found. Consider using LSPErrorCodes.RequestFailed or LSPErrorCodes.InternalError instead.

Suggested change

	LSPErrorCodes.ServerCancelled,
	LSPErrorCodes.RequestFailed,