Fix basedpyright install failure on Debian

Debian ships venv's ensurepip in a separate python3-venv package. Without
it, `python3 -m venv` exits 1, so mason cannot build the venv it installs
basedpyright into, and mason-lspconfig reports a failed install on every
startup.

Add python3-venv to the apt package list, and guard the basedpyright entry
in ensure_installed the same way the npm-sourced servers are already
guarded, so a machine missing it gets one warning instead of a failed
install every time.

config/nvim/lua/plugins/lsp.lua

@@ -1,4 +1,21 @@
-local servers = { "basedpyright", "ruff" }
+-- ruff is a prebuilt GitHub release binary, but mason installs basedpyright
+-- from PyPI into a venv. Debian ships venv's ensurepip separately
+-- (python3-venv), and without it every startup fails the install.
+local servers = { "ruff" }
+
+local function has_python_venv()
+  if vim.fn.executable("python3") ~= 1 then
+    return false
+  end
+  vim.fn.system({ "python3", "-c", "import ensurepip" })
+  return vim.v.shell_error == 0
+end
+
+if has_python_venv() then
+  table.insert(servers, "basedpyright")
+else
+  vim.notify("python3 venv unavailable (install python3-venv): skipping LSP server basedpyright", vim.log.levels.WARN)
+end
 
 local npm_servers = { "ts_ls", "svelte", "html", "cssls", "bashls" }
 local function has_public_npm()

packages/apt.global

@@ -6,6 +6,9 @@ powerline
 thefuck
 tree
 nodejs
+# Debian splits venv's ensurepip out of python3; mason needs `python3 -m venv`
+# to install PyPI-sourced LSP servers (basedpyright)
+python3-venv
 zsh
 htop
 jq