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/coderd.go

@@ -1480,6 +1480,7 @@ func New(options *Options) *API {
 							})
 						})
 						r.Route("/webpush", func(r chi.Router) {
+							r.Use(httpmw.RequireExperimentWithDevBypass(api.Experiments, codersdk.ExperimentWebPush))
 							r.Post("/subscription", api.postUserWebpushSubscription)
 							r.Delete("/subscription", api.deleteUserWebpushSubscription)
 							r.Post("/test", api.postUserPushNotificationTest)