feat(v2/postgres): add QueryExecMode config with SimpleProtocol default for PgBouncer

Context

Part of the LabKit v2 Artifact Registry follow-up work. Identified as a required change in the Artifact Registry PoC assessment.

Tracking issue: gitlab-org/quality/quality-engineering/team-tasks#4291 (closed) Parent epic: gitlab-org/quality#360

What's in this MR?

• Expose QueryExecMode as a configuration field on postgres.Config
• Default to pgx.QueryExecModeSimpleProtocol, which avoids server-side prepared statements that break under PgBouncer transaction pooling (the standard GitLab.com deployment)
• Add QueryExecMode() accessor method on Client for observability and testing
• Update PgBouncer documentation in doc.go and README.md to explain the default and how to override it

pgx defaults to QueryExecModeCacheStatement which uses PostgreSQL prepared statements. PgBouncer in transaction-pooling mode cannot track prepared statements across connections, causing prepared statement "..." already exists errors. The Container Registry already uses QueryExecModeSimpleProtocol to solve this.

Manual PgBouncer Verification

To verify locally that SimpleProtocol works through PgBouncer:

# 1. Start PostgreSQL + PgBouncer (transaction pooling mode)
docker run -d --name pg-test \
  -e POSTGRES_PASSWORD=test -e POSTGRES_USER=postgres -e POSTGRES_DB=testdb \
  -p 5432:5432 postgres:16-alpine

docker run -d --name pgbouncer-test --link pg-test \
  -e DATABASE_URL="postgres://postgres:test@pg-test:5432/testdb" \
  -e POOL_MODE=transaction \
  -e AUTH_TYPE=plain \
  -p 6432:5432 edoburu/pgbouncer:latest

# 2. Wait for services to be ready
until docker exec pg-test pg_isready -U postgres; do sleep 1; done

# 3. Verify SimpleProtocol works through PgBouncer
docker exec pg-test psql -U postgres -d testdb -c "SELECT 1+1"
docker exec -e PGPASSWORD=test pgbouncer-test psql -U postgres -d testdb -h 127.0.0.1 -p 5432 -c "SELECT 1+1" \
  && echo "PgBouncer connection OK" || echo "PgBouncer connection failed"

# 4. Cleanup
docker rm -f pg-test pgbouncer-test

Automated integration tests (PostgreSQL + PgBouncer CI services) are in the follow-up !364 (merged).

Related

• gitlab-org/quality/quality-engineering/team-tasks#4291 (closed)