refactor(agent): update agentcontainers api initialization (#17600)

There were too many ways to configure the agentcontainers API resulting
in inconsistent behavior or features not being enabled. This refactor
introduces a control flag for enabling or disabling the containers API.
When disabled, all implementations are no-op and explicit endpoint
behaviors are defined. When enabled, concrete implementations are used
by default but can be overridden by passing options.

coderd/workspaceagents.go

@@ -848,6 +848,11 @@ func (api *API) workspaceAgentListContainers(rw http.ResponseWriter, r *http.Req
 			})
 			return
 		}
+		// If the agent returns a codersdk.Error, we can return that directly.
+		if cerr, ok := codersdk.AsError(err); ok {
+			httpapi.Write(ctx, rw, cerr.StatusCode(), cerr.Response)
+			return
+		}
 		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
 			Message: "Internal error fetching containers.",
 			Detail:  err.Error(),