Add integration tests that verify QueryExecModeSimpleProtocol works
through PgBouncer in transaction-pooling mode. Tests are gated behind
the 'integration' build tag and require live PostgreSQL + PgBouncer
instances.

Includes:
- docker-compose.yml for local development
- CI job with PostgreSQL and PgBouncer as services
- Tests covering direct PostgreSQL and PgBouncer paths
- Parameterized query test through the pooler

- Use t.Cleanup with context.Background() for client shutdown instead
  of defer with the test context (which may be cancelled by teardown)
- Add readiness wait loop in CI job for PostgreSQL and PgBouncer
  services before running tests

The edoburu/pgbouncer image uses v-prefixed tags with a patch suffix
(e.g. v1.23.1-p3), not bare version numbers. Tag 1.23.1 does not
exist, which caused the CI pipeline to fail pulling the image.

The job is under the integration stage, so the prefix is redundant.

# Conflicts:
#   .gitlab-ci.yml

test(v2/postgres): add PgBouncer integration tests with CI job

See merge request !364



Merged-by: Doug Barrett <dbarrett@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>