feat(lsp): support connect via named pipes/unix domain sockets (#26032)

Closes https://github.com/neovim/neovim/issues/26031

Co-authored-by: Mathias Fussenegger <f.mathias@zignar.net>

test/functional/plugin/lsp_spec.lua

@@ -3861,6 +3861,39 @@ describe('LSP', function()
       ]]
       eq(result.method, "initialize")
     end)
+    it('can connect to lsp server via rpc.domain_socket_connect', function()
+      local tmpfile
+      if is_os("win") then
+        tmpfile = "\\\\.\\\\pipe\\pipe.test"
+        else
+        tmpfile = helpers.tmpname()
+        os.remove(tmpfile)
+      end
+      local result = exec_lua([[
+        local SOCK = ...
+        local uv = vim.uv
+        local server = uv.new_pipe(false)
+        server:bind(SOCK)
+        local init = nil
+
+        server:listen(127, function(err)
+                assert(not err, err)
+                local client = uv.new_pipe()
+                server:accept(client)
+                client:read_start(require("vim.lsp.rpc").create_read_loop(function(body)
+                        init = body
+                        client:close()
+                end))
+        end)
+        vim.lsp.start({ name = "dummy", cmd = vim.lsp.rpc.domain_socket_connect(SOCK) })
+        vim.wait(1000, function() return init ~= nil end)
+        assert(init, "server must receive `initialize` request")
+        server:close()
+        server:shutdown()
+        return vim.json.decode(init)
+      ]], tmpfile)
+      eq(result.method, "initialize")
+    end)
   end)
 
   describe('handlers', function()