fix: LSP format deadlock, null diagnostics, stale state after rename, and simplify code

- Fix self-deadlock in format(): NotifyFileChange acquires h.mu but
  format() already holds it. Extract notifyFileChangeLocked() and use
  it from both format() and applyWorkspaceEdit()
- Fix codeActions() nil diagnostics slice producing JSON null instead of []
- Fix applyWorkspaceEdit() not notifying LSP server of file changes after
  rename, causing stale diagnostics/hover results
- Simplify tool registration by replacing lspToolDef struct with lspTool()
  factory function
- Deduplicate definition/implementations into shared locationRequest() helper
- Extract initializeLocked() from ensureInitialized() for readability

Assisted-By: docker-agent

Author: dgageot