refactor: convert workspacesdk.AgentConn to an interface (#19392)

Fixes https://github.com/coder/internal/issues/907

We convert `workspacesdk.AgentConn` to an interface and generate a mock
for it. This allows writing `coderd` tests that rely on the agent's HTTP
api to not have to set up an entire tailnet networking stack.

coderd/coderd.go

@@ -325,6 +325,9 @@ func New(options *Options) *API {
 		})
 	}
 
+	if options.PrometheusRegistry == nil {
+		options.PrometheusRegistry = prometheus.NewRegistry()
+	}
 	if options.Authorizer == nil {
 		options.Authorizer = rbac.NewCachingAuthorizer(options.PrometheusRegistry)
 		if buildinfo.IsDev() {
@@ -381,9 +384,6 @@ func New(options *Options) *API {
 	if options.FilesRateLimit == 0 {
 		options.FilesRateLimit = 12
 	}
-	if options.PrometheusRegistry == nil {
-		options.PrometheusRegistry = prometheus.NewRegistry()
-	}
 	if options.Clock == nil {
 		options.Clock = quartz.NewReal()
 	}