refactor(webpush): use RequireExperimentWithDevBypass middleware (#22525)

Replace manual experiment checks in web-push handlers with the
`RequireExperimentWithDevBypass` middleware on the route group, matching
the pattern used by OAuth2, Agents, and MCP experiments.

## Changes

- **`coderd/coderd.go`**: Add `RequireExperimentWithDevBypass`
middleware to `/webpush` route group
- **`coderd/webpush.go`**: Remove inline
`api.Experiments.Enabled(codersdk.ExperimentWebPush)` checks from all
three handlers
- **`cli/server.go`**: Gate webpush dispatcher initialization with
`buildinfo.IsDev()` fallback so dev builds always init the real
dispatcher
- **`coderd/webpush_test.go`**: Remove experiment enablement from tests
(dev bypass handles it)

Net effect: -26 lines removed, +5 added.

Created using whatchamacallits (Opus 4.6 Max)

coderd/webpush_test.go

@@ -30,11 +30,7 @@ func TestWebpushSubscribeUnsubscribe(t *testing.T) {
 
 	ctx := testutil.Context(t, testutil.WaitShort)
 
-	dv := coderdtest.DeploymentValues(t)
-	dv.Experiments = []string{string(codersdk.ExperimentWebPush)}
-	client := coderdtest.New(t, &coderdtest.Options{
-		DeploymentValues: dv,
-	})
+	client := coderdtest.New(t, &coderdtest.Options{})
 	owner := coderdtest.CreateFirstUser(t, client)
 	memberClient, _ := coderdtest.CreateAnotherUser(t, client, owner.OrganizationID)
 	_, anotherMember := coderdtest.CreateAnotherUser(t, client, owner.OrganizationID)
@@ -112,12 +108,9 @@ func TestDeleteWebpushSubscription(t *testing.T) {
 		store, ps := dbtestutil.NewDB(t)
 		wrappedStore := &testWebpushErrorStore{Store: store}
 
-		dv := coderdtest.DeploymentValues(t)
-		dv.Experiments = []string{string(codersdk.ExperimentWebPush)}
 		client := coderdtest.New(t, &coderdtest.Options{
-			DeploymentValues: dv,
-			Database:         wrappedStore,
-			Pubsub:           ps,
+			Database: wrappedStore,
+			Pubsub:   ps,
 		})
 		owner := coderdtest.CreateFirstUser(t, client)
 		memberClient, _ := coderdtest.CreateAnotherUser(t, client, owner.OrganizationID)