fix(lsp): use root_dir as cmd CWD #40331

Problem:
cmd given as string[] always starts using Nvim's CWD, which is arbitrary.

Solution:
If cmd_cwd is not given, use root_dir as CWD.

BREAKING CHANGE: LSP commands given as string arrays now use `root_dir` as
the process working directory when `cmd_cwd` is unset.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/plugin/lsp_spec.lua

@@ -2932,6 +2932,52 @@ describe('LSP', function()
   end)
 
   describe('cmd', function()
+    it('cmd string[] CWD', function()
+      local root_dir = tmpname(false)
+      local cmd_cwd = tmpname(false)
+      mkdir(root_dir)
+      mkdir(cmd_cwd)
+
+      local cwd = exec_lua(function()
+        return assert(vim.uv.cwd())
+      end)
+
+      local cases = {
+        {
+          desc = 'cmd_cwd takes precedence',
+          config = { name = 'cwd-test-cmd-cwd', root_dir = root_dir, cmd_cwd = cmd_cwd },
+          expected_cwd = cmd_cwd,
+        },
+        {
+          desc = 'root_dir is used when cmd_cwd is unset',
+          config = { name = 'cwd-test-root-dir', root_dir = root_dir },
+          expected_cwd = root_dir,
+        },
+        {
+          desc = 'current cwd is used when cmd_cwd and root_dir are unset',
+          config = { name = 'cwd-test-current-cwd' },
+          expected_cwd = cwd,
+        },
+      }
+
+      for _, case in ipairs(cases) do
+        local outfile = tmpname(false)
+        local config = vim.tbl_extend('force', {
+          cmd = {
+            n.nvim_prog,
+            '--clean',
+            '--headless',
+            ('+call writefile([getcwd()], %q)'):format(outfile),
+            '+qa!',
+          },
+        }, case.config)
+        exec_lua(function(conf)
+          assert(vim.lsp.start(conf, { attach = false }))
+        end, config)
+        t.assert_log(vim.pesc(case.expected_cwd), outfile)
+      end
+    end)
+
     it('connects to lsp server via rpc.connect using ip address', function()
       exec_lua(create_tcp_echo_server)
       exec_lua(function()