fix: replace moby/moby namesgenerator with internal implementation (#21377)

Replace the external moby/moby/pkg/namesgenerator dependency with an
internal implementation using gofakeit/v7. The moby package has ~25k
unique name combinations, and with its retry parameter only adds a
random digit 0-9, giving ~250k possibilities. In parallel tests, this
has led to collisions (flakes).

The new internal API at coderd/util/namesgenerator eliminates the
external dependnecy and offers functions with explicit uniqueness
guarantees. This PR also consolidates fragmented name generation in a
few places to use the new package.

| Old (moby/moby)                     | New                    |
|-------------------------------------|------------------------|
| namesgenerator.GetRandomName(0)     | NameWith("_")          |
| namesgenerator.GetRandomName(>0)    | NameDigitWith("_")     |
| testutil.GetRandomName(t)           | UniqueName()           |
| testutil.GetRandomNameHyphenated(t) | UniqueNameWith("-")    |

namesgenerator package API:
- NameWith(delim): random name, not unique
- NameDigitWith(delim): random name with 1-9 suffix, not unique
- UniqueName(): guaranteed unique via atomic counter
- UniqueNameWith(delim): unique with custom delimiter

Names continue to be docker style `[adjective][delim][surname]`. Unique
names are truncated to 32 characters (preserving the numeric suffix) to
fit common name length limits in Coder.

Related test flakes:
https://github.com/coder/internal/issues/1212
https://github.com/coder/internal/issues/118
https://github.com/coder/internal/issues/1068

coderd/coderdtest/coderdtest_test.go

@@ -1,8 +1,11 @@
 package coderdtest_test
 
 import (
+	"strings"
 	"testing"
+	"unicode"
 
+	"github.com/stretchr/testify/require"
 	"go.uber.org/goleak"
 
 	"github.com/coder/coder/v2/coderd/coderdtest"
@@ -28,3 +31,22 @@ func TestNew(t *testing.T) {
 	_, _ = coderdtest.NewGoogleInstanceIdentity(t, "example", false)
 	_, _ = coderdtest.NewAWSInstanceIdentity(t, "an-instance")
 }
+
+func TestRandomName(t *testing.T) {
+	t.Parallel()
+
+	for range 10 {
+		name := coderdtest.RandomName(t)
+
+		require.NotEmpty(t, name, "name should not be empty")
+		require.NotContains(t, name, "_", "name should not contain underscores")
+
+		// Should be title cased (e.g., "Happy Einstein").
+		words := strings.Split(name, " ")
+		require.Len(t, words, 2, "name should have exactly two words")
+		for _, word := range words {
+			firstRune := []rune(word)[0]
+			require.True(t, unicode.IsUpper(firstRune), "word %q should start with uppercase letter", word)
+		}
+	}
+}