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)