refactor: consolidate chat streaming endpoints under /stream (#23248)

Moves per-chat streaming/watch endpoints under a `/stream` sub-route for
better API consistency:

| Before | After |
|--------|-------|
| `GET /{chat}/stream` | `GET /{chat}/stream/` |
| `GET /{chat}/desktop` | `GET /{chat}/stream/desktop` |
| `GET /{chat}/git/watch` | `GET /{chat}/stream/git` |

### Changes
- **`coderd/coderd.go`** — Route definitions: replaced flat routes with
`r.Route("/stream", ...)` sub-router
- **`site/src/api/api.ts`** — Updated WebSocket URLs for `watchChatGit`
and `watchChatDesktop`
- **`coderd/chats_test.go`** — Updated desktop test URL
- **`coderd/workspaceagents_internal_test.go`** — Updated git watcher
test URLs (route mounts + dial URLs)
- **`site/src/pages/AgentsPage/AgentDetail.stories.tsx`** — Updated
storybook WebSocket mock paths

coderd/coderd.go

@@ -1209,13 +1209,15 @@ func New(options *Options) *API {
 			r.Route("/{chat}", func(r chi.Router) {
 				r.Use(httpmw.ExtractChatParam(options.Database))
 				r.Get("/", api.getChat)
-				r.Get("/git/watch", api.watchChatGit)
-				r.Get("/desktop", api.watchChatDesktop)
 				r.Patch("/", api.patchChat)
 				r.Get("/messages", api.getChatMessages)
 				r.Post("/messages", api.postChatMessages)
 				r.Patch("/messages/{message}", api.patchChatMessage)
-				r.Get("/stream", api.streamChat)
+				r.Route("/stream", func(r chi.Router) {
+					r.Get("/", api.streamChat)
+					r.Get("/desktop", api.watchChatDesktop)
+					r.Get("/git", api.watchChatGit)
+				})
 				r.Post("/interrupt", api.interruptChat)
 				r.Get("/diff", api.getChatDiffContents)
 				r.Route("/queue/{queuedMessage}", func(r chi.Router) {