test: hold three edits the CI mutation sample found unrefused

The pull request's mutate job, on the docker8 fleet, left three edits
no unit test refused: isTerminalWriter's file-mode check (only a buffer
was tried), singleContainerRemoteUserUID with an image inspection that
has no Config, and win32's ${localWorkspaceFolderBasename}. A local run
refused them only through Docker-backed tests the job does not reach.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: aledbf

internal/cli/check_terminal_held_test.go

@@ -0,0 +1,28 @@
+package cli
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// A character device, as a terminal is, counts as a terminal writer; a regular
+// file does not.
+func TestIsTerminalWriterChecksTheFileMode(t *testing.T) {
+	devNull, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer devNull.Close()
+	if !isTerminalWriter(devNull) {
+		t.Errorf("%s is a character device: want a terminal writer", os.DevNull)
+	}
+	regular, err := os.Create(filepath.Join(t.TempDir(), "out"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer regular.Close()
+	if isTerminalWriter(regular) {
+		t.Error("a regular file is not a terminal writer")
+	}
+}

internal/cli/up_remote_uid_held_test.go

@@ -0,0 +1,29 @@
+package cli
+
+import (
+	"io"
+	"testing"
+
+	"github.com/devcontainers/cli/internal/config"
+	"github.com/devcontainers/cli/internal/docker"
+	"github.com/devcontainers/cli/internal/log"
+)
+
+// An image whose inspection has no Config carries no metadata: the UID update
+// reads none and, with updates off, keeps the image.
+func TestSingleContainerRemoteUserUIDImageWithoutConfig(t *testing.T) {
+	logger := log.New(log.Options{Writer: io.Discard})
+	engine := &fakeEngineAPI{images: map[string]string{"img:1": `{}`}}
+	r := &upRunner{
+		log:    logger,
+		engine: &docker.EngineClient{API: engine, Log: logger},
+		opts:   &upOpts{updateRemoteUserUIDDefault: "off"},
+	}
+	got, err := r.singleContainerRemoteUserUID(t.Context(), "img:1", &config.DevContainer{}, t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "img:1" {
+		t.Errorf("image = %q, want img:1 kept", got)
+	}
+}

internal/config/varsub_win_held_test.go

@@ -0,0 +1,23 @@
+package config
+
+import "testing"
+
+// On win32 both '\' and '/' separate the workspace folder's basename, and a
+// folder without one is its own basename.
+func TestLocalWorkspaceFolderBasenameOnWindows(t *testing.T) {
+	for folder, want := range map[string]string{
+		`C:\Users\me\project`: "project",
+		`C:/Users/me/project`: "project",
+		`C:\Users/me\mixed`:   "mixed",
+		`project`:             "project",
+	} {
+		ctx := HostSubContext{Platform: "win32", LocalWorkspaceFolder: folder, Env: map[string]string{}}
+		got, err := NewVariableResolver().Resolve(SubstitutionContext{HostSubContext: ctx}, PhaseHost, "${localWorkspaceFolderBasename}")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got != want {
+			t.Errorf("basename of %q = %v, want %q", folder, got, want)
+		}
+	}
+}