feat(cli): add experimental rpty command (#16700)

Relates to https://github.com/coder/coder/issues/16419

Builds upon https://github.com/coder/coder/pull/16638 and adds a command
`exp rpty` that allows you to open a ReconnectingPTY session to an
agent.

This ultimately allows us to add an integration-style CLI test to verify
the functionality added in #16638 .

cli/exp_errors_test.go

@@ -0,0 +1,93 @@
+package cli_test
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/cli"
+	"github.com/coder/coder/v2/cli/clitest"
+	"github.com/coder/serpent"
+)
+
+type commandErrorCase struct {
+	Name string
+	Cmd  []string
+}
+
+// TestErrorExamples will test the help output of the
+// coder exp example-error using golden files.
+func TestErrorExamples(t *testing.T) {
+	t.Parallel()
+
+	coderRootCmd := getRoot(t)
+
+	var exampleErrorRootCmd *serpent.Command
+	coderRootCmd.Walk(func(command *serpent.Command) {
+		if command.Name() == "example-error" {
+			// cannot abort early, but list is small
+			exampleErrorRootCmd = command
+		}
+	})
+	require.NotNil(t, exampleErrorRootCmd, "example-error command not found")
+
+	var cases []commandErrorCase
+
+ExtractCommandPathsLoop:
+	for _, cp := range extractCommandPaths(nil, exampleErrorRootCmd.Children) {
+		cmd := append([]string{"exp", "example-error"}, cp...)
+		name := fmt.Sprintf("coder %s", strings.Join(cmd, " "))
+		for _, tt := range cases {
+			if tt.Name == name {
+				continue ExtractCommandPathsLoop
+			}
+		}
+		cases = append(cases, commandErrorCase{Name: name, Cmd: cmd})
+	}
+
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.Name, func(t *testing.T) {
+			t.Parallel()
+
+			var outBuf bytes.Buffer
+
+			coderRootCmd := getRoot(t)
+
+			inv, _ := clitest.NewWithCommand(t, coderRootCmd, tt.Cmd...)
+			inv.Stderr = &outBuf
+			inv.Stdout = &outBuf
+
+			err := inv.Run()
+
+			errFormatter := cli.NewPrettyErrorFormatter(&outBuf, false)
+			errFormatter.Format(err)
+
+			clitest.TestGoldenFile(t, tt.Name, outBuf.Bytes(), nil)
+		})
+	}
+}
+
+func extractCommandPaths(cmdPath []string, cmds []*serpent.Command) [][]string {
+	var cmdPaths [][]string
+	for _, c := range cmds {
+		cmdPath := append(cmdPath, c.Name())
+		cmdPaths = append(cmdPaths, cmdPath)
+		cmdPaths = append(cmdPaths, extractCommandPaths(cmdPath, c.Children)...)
+	}
+	return cmdPaths
+}
+
+// Must return a fresh instance of cmds each time.
+func getRoot(t *testing.T) *serpent.Command {
+	t.Helper()
+
+	var root cli.RootCmd
+	rootCmd, err := root.Command(root.AGPL())
+	require.NoError(t, err)
+
+	return rootCmd
+}