chore: switch agent gone response from 502 to 404 (#23090)

When a user creates a workspace, opens the web terminal, then the
workspace stops but the web terminal remains open the web terminal will
retry the connection. Coder will issue a HTTP 502 Bad Gateway response
when this occurs because coderd cannot connect to the workspace agent,
however this is problematic as any load balancer sitting in front of
Coder sees a 502 and thinks Coder is unhealthy.

The main change is in
https://github.com/coder/coder/pull/23090/changes#diff-bbe3b56ed3532289481a0e977867cd15048b7ca718ce676aae3f3332378eebc2R97,
however the main test and downstream tests are also updated.

This PR changes the response to a [HTTP
404](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status/404)
after internal discussion.

<img width="1832" height="1511" alt="image"
src="https://github.com/user-attachments/assets/0baff80d-bb98-4644-89cd-e80c87947098"
/>

Created with the help of Mux, reviewed and tested by a human

coderd/workspaceapps/db_test.go

@@ -1016,7 +1016,7 @@ func Test_ResolveRequest(t *testing.T) {
 
 		w := rw.Result()
 		defer w.Body.Close()
-		require.Equal(t, http.StatusBadGateway, w.StatusCode)
+		require.Equal(t, http.StatusNotFound, w.StatusCode)
 		assertConnLogContains(t, rw, r, connLogger, workspace, agentNameUnhealthy, appNameAgentUnhealthy, database.ConnectionTypeWorkspaceApp, me.ID)
 		require.Len(t, connLogger.ConnectionLogs(), 1)
 

coderd/workspaceapps/errors.go

@@ -77,7 +77,7 @@ func WriteWorkspaceApp500(log slog.Logger, accessURL *url.URL, rw http.ResponseW
 	})
 }
 
-// WriteWorkspaceAppOffline writes a HTML 502 error page for a workspace app. If
+// WriteWorkspaceAppOffline writes a HTML 404 error page for a workspace app. If
 // appReq is not nil, it will be used to log the request details at debug level.
 func WriteWorkspaceAppOffline(log slog.Logger, accessURL *url.URL, rw http.ResponseWriter, r *http.Request, appReq *Request, msg string) {
 	if appReq != nil {
@@ -94,7 +94,7 @@ func WriteWorkspaceAppOffline(log slog.Logger, accessURL *url.URL, rw http.Respo
 	}
 
 	site.RenderStaticErrorPage(rw, r, site.ErrorPageData{
-		Status:      http.StatusBadGateway,
+		Status:      http.StatusNotFound,
 		Title:       "Application Unavailable",
 		Description: msg,
 		Actions: []site.Action{