flake: TestExternalAuthCallback/ValidateURL

[johnstcn]

		// If the validation URL says unauthorized, the callback
		// URL to re-authenticate should be returned.
		srv.Config.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			w.WriteHeader(http.StatusUnauthorized)
		})
		res, err := agentClient.ExternalAuth(ctx, agentsdk.ExternalAuthRequest{
			Match: "github.com/asd/asd",
		})
		require.NoError(t, err) <-- here
		require.NotEmpty(t, res.URL)

Seen here: https://github.com/coder/coder/actions/runs/16619855254/job/47021646753

t.go:106: 2025-07-30 10:26:39.319 [warn]  coderd: GET  host=localhost:45915  path=/api/v2/workspaceagents/me/external-auth  proto=HTTP/1.1  remote_addr=127.0.0.1  start="2025-07-30T10:26:39.074728355Z"  requestor_id=b2fb915d-5244-411b-a22c-4f71f822bdaa  requestor_name=testuser  requestor_email=testuser@coder.com  took=244.858143ms  status_code=500  latency_ms=244  request_id=3160f010-4057-4aa6-935b-2656df15e747 ...
        response_body= {
                       	"message": "Failed to refresh external auth token.",
                       	"detail": "validate external auth token: Get \"[http://127.0.0.1:38511\](http://127.0.0.1:38511/)": net/http: HTTP/1.x transport connection broken: http: CloseIdleConnections called"
                       }
    externalauth_test.go:582: 
        	Error Trace:	/home/runner/work/coder/coder/coderd/externalauth_test.go:582
        	Error:      	Received unexpected error:
        	            	GET http://localhost:45915/api/v2/workspaceagents/me/external-auth?id=&match=github.com%2Fasd%2Fasd: unexpected status code 500: Failed to refresh external auth token.
        	            		Error: validate external auth token: Get "http://127.0.0.1:38511/": net/http: HTTP/1.x transport connection broken: http: CloseIdleConnections called
        	Test:       	TestExternalAuthCallback/ValidateURL