refactor: deduplicate utility helpers across the codebase (#23338)

Audited exported helpers in `coderd/util/*`, `testutil`, `cryptorand`,
and friends, then replaced duplicated implementations with canonical
versions.

- **fix: `maps.SortedKeys` generic signature** — value type was
hardcoded to `any`, making it impossible to actually call. Added second
type parameter `V any`. Added table-driven tests with `cmp.Diff`.
- **refactor: replace ad-hoc ptr helpers with `ptr.Ref`** — removed
`int64Ptr`, `stringPtr`, `boolPtr`, `i64ptr`, `strPtr`, `PtrInt32`
across 6 files.
- **refactor: replace local `sortedKeys`/`sortKeys` with
`maps.SortedKeys`** — now that the signature is fixed, scripts can use
it.
- **refactor: replace hand-rolled `capitalize` with
`strings.Capitalize`** — the typegen version was also not UTF-8 safe.

> 🤖 This PR was created with the help of Coder Agents, and was reviewed
by my human. 🧑‍💻

coderd/userauth_test.go

@@ -43,6 +43,7 @@ import (
 	"github.com/coder/coder/v2/coderd/notifications"
 	"github.com/coder/coder/v2/coderd/notifications/notificationstest"
 	"github.com/coder/coder/v2/coderd/promoauth"
+	"github.com/coder/coder/v2/coderd/util/ptr"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/cryptorand"
 	"github.com/coder/coder/v2/testutil"
@@ -405,7 +406,7 @@ func TestUserOAuth2Github(t *testing.T) {
 				AuthenticatedUser: func(ctx context.Context, _ *http.Client) (*github.User, error) {
 					return &github.User{
 						AvatarURL: github.String("/hello-world"),
-						ID:        i64ptr(1234),
+						ID:        ptr.Ref[int64](1234),
 						Login:     github.String("kyle"),
 						Name:      github.String("Kylium Carbonate"),
 					}, nil
@@ -473,7 +474,7 @@ func TestUserOAuth2Github(t *testing.T) {
 				AuthenticatedUser: func(_ context.Context, _ *http.Client) (*github.User, error) {
 					return &github.User{
 						AvatarURL: github.String("/hello-world"),
-						ID:        i64ptr(1234),
+						ID:        ptr.Ref[int64](1234),
 						Login:     github.String("kyle"),
 						Name:      github.String(" " + strings.Repeat("a", 129) + " "),
 					}, nil
@@ -2525,10 +2526,6 @@ func oauth2Callback(t *testing.T, client *codersdk.Client, opts ...func(*http.Re
 	return res
 }
 
-func i64ptr(i int64) *int64 {
-	return &i
-}
-
 func authCookieValue(cookies []*http.Cookie) string {
 	for _, cookie := range cookies {
 		if cookie.Name == codersdk.SessionTokenCookie {