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/coderd_test.go

@@ -384,9 +384,9 @@ func TestCSRFExempt(t *testing.T) {
 		data, _ := io.ReadAll(resp.Body)
 		_ = resp.Body.Close()
 
-		// A StatusBadGateway means Coderd tried to proxy to the agent and failed because the agent
+		// A StatusNotFound means Coderd tried to proxy to the agent and failed because the agent
 		// was not there. This means CSRF did not block the app request, which is what we want.
-		require.Equal(t, http.StatusBadGateway, resp.StatusCode, "status code 500 is CSRF failure")
+		require.Equal(t, http.StatusNotFound, resp.StatusCode, "status code 500 is CSRF failure")
 		require.NotContains(t, string(data), "CSRF")
 	})
 }