feat: add REST API handlers and client methods for user secrets (#24107)

Add the five REST endpoints for managing user secrets, SDK client
methods, and handler tests.

Endpoints:
- `POST /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets/{name}`
- `PATCH /api/v2/users/{user}/secrets/{name}`
- `DELETE /api/v2/users/{user}/secrets/{name}`

Routes are registered under the existing `/{user}` group with
`ExtractUserParam`. The delete query was changed from `:exec` to
`:execrows` so the handler can distinguish "not found" from success
(DELETE with `:exec` silently returns nil for zero affected rows).

coderd/coderd.go

@@ -1608,6 +1608,15 @@ func New(options *Options) *API {
 
 						r.Get("/gitsshkey", api.gitSSHKey)
 						r.Put("/gitsshkey", api.regenerateGitSSHKey)
+						r.Route("/secrets", func(r chi.Router) {
+							r.Post("/", api.postUserSecret)
+							r.Get("/", api.getUserSecrets)
+							r.Route("/{name}", func(r chi.Router) {
+								r.Get("/", api.getUserSecret)
+								r.Patch("/", api.patchUserSecret)
+								r.Delete("/", api.deleteUserSecret)
+							})
+						})
 						r.Route("/notifications", func(r chi.Router) {
 							r.Route("/preferences", func(r chi.Router) {
 								r.Get("/", api.userNotificationPreferences)