lsp: Fix vtsls completeFunctionCalls setting being ignored

[yeskunall]

Some language servers (notably vtsls with completeFunctionCalls) update insertText/textEdit during completionItem/resolve to add snippet content like function call parentheses. Previously, Completion.new_text was only set from the initial completion response and never re-derived after resolve, so the snippet text was silently discarded.

Re-derive new_text from the resolved lsp_completion. Only the text content is updated -- replace/insert ranges are left as anchors from the original response since the LSP ranges in the resolved text_edit are stale when completions are cached across keystrokes (re: #34094).

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #53275.

Release Notes:

• Fixed the vtsls completeFunctionCalls setting being ignored

[smitbarmase]

Looks good overall. Since this still isn’t LSP-spec-correct, we should probably keep that comment.

// We must not use any data such as sortText, filterText, insertText and textEdit to edit Completion since they are not suppose change during resolve.
// Refer: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion

Also, let’s add a comment referencing the underlying resolve request in typescript-language-features#L218 which vtsls simply wraps.

This fix should be treated as a workaround for this specific VSCode TypeScript extension case.