Upgrade `vscode-languageclient` v8 → v9 (+ matching server package)

[GrahamStrickland]

Bump vscode-languageclient (client) and vscode-languageserver (server) from v8 to v9. The LSP spec has continued to evolve; newer features include:

• Position encodings (important for non-UTF-16 clients like Neovim)
• Inline completions
• Refined semantic-tokens behaviour

Acceptance:

• client/package.json and server/package.json updated
• Any breaking-change call sites adapted
• Build + smoke test in VSCode and Neovim still works

Source: IMPROVEMENTS.md → Upgrade vscode-languageclient v8 → v9

---

Filed by Claude (Opus 4.7) on behalf of @GrahamStrickland.