chore(deps): take vscode-languageserver-protocol 3.18 with vscode-jsonrpc 9 #1203

Split out of #987.

vscode-languageserver-protocol 3.18.x changes Diagnostic.message to string | MarkupContent, and it depends on vscode-jsonrpc 9, while the app pins vscode-jsonrpc ^8.2.1 (package.json). That would put two copies side by side, and Dependabot never proposes the jsonrpc major.

Change

• src/main/lspManager.ts, around line 1125 (message: diagnostic.message): narrow with typeof m === 'string' ? m : m.value.
• Bump vscode-jsonrpc to 9 in the same PR.
• Land it after fix(lsp): order document intent, and stop discarding a request's own sync #1108, which also edits lspManager.ts.

Acceptance

• npm ls vscode-jsonrpc shows one copy.
• The LSP tests pass.
• Diagnostics whose message is Markdown render as text, not [object Object].