ci: skip the GCP load balancer test on Release PRs too

[mogita]

Ticket

CHA-5511

Problem

#89 made four of this repo's five test jobs skip on a release-please Release PR. integration-gcp-lb was left out on the grounds that it is a keep-alive probe rather than a gate, so skipping it would cost an exercise of the load balancer path.

That reasoning was wrong. spec/integration/gcp_lb_keepalive_spec.rb is an ordinary regression test for CHA-4943:

# CHA-4943: GCP SSL-proxy LBs close idle TLS around ~30s. The SDK pool must
# drop connections before that, otherwise the next request raises
# GetStreamRuby::TransportError (SSL_read: unexpected eof while reading).

it 'reuses the pooled client after 35s idle without TLS EOF' do
  ids, = create_test_users(1)
  sleep ENV.fetch('STREAM_GCP_LB_IDLE_SLEEP', '35').to_i
  expect { create_test_users(1) }.not_to raise_error

"Keep-alive" is HTTP connection reuse, not warming anything. It tests the SDK's connection-pool idle timeout, 35 of its 71 seconds are a hardcoded sleep, and a Release PR changes no SDK code.

Solution

Give it the same guard the other four jobs carry, and drop the comment that explained the exception.

Result

All five test jobs now behave identically on a Release PR. The wall time drops from 71s to 58s, which is this repo's CodeQL floor and cannot go lower.

How to verify

1. This PR is not a Release PR, so GCP load balancer keep-alive should run as usual.
2. On the next Release PR, all five test jobs should report Skipped.