LSP compliance textDocument/completion

Plugins like nvim-cmp provide features such as a completion menu and ghost text by utilizing the standard `textDocument/completion` command, as specified in the LSP documentation:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion. Currently, the command in use is `llm-ls/getCompletions`, but if we can ensure compliance with the LSP spec, it would unlock the full range of customization options offered by completion plugins.

Start by comparing the existing `llm-ls/getCompletions` handling with the LSP `textDocument/completion` specification linked in the issue. Trace the completion request entry point and determine the response shape required by the standard; done means completion plugins can use the standard command and its customization features.

Written by the indexing model from the issue text.