fix: support base-image extension dependencies

[ardentperf]

Current code doesn't work if an extension dependency is in the base image. I ran into this while working on pg-stat-kcache and pgsentinel which both depend on pg_stat_statements.

We need to support both models - some dependencies are not in the base image (eg. pgrouting or mobilitydb)

Proposed behavior

Base-image dependencies use an explicit prefix in the existing
required_extensions field:

required_extensions = ["base-image:pg_stat_statements"]

For such a dependency:

1. It is omitted from the generated extension image list.
2. It remains in the generated database extension specification so the test database installs it from the base image.

Unprefixed dependencies retain the current local-repository behavior, and a missing unprefixed dependency remains an error.

Validation

• DAGGER_SESSION_PORT=1 DAGGER_SESSION_TOKEN=test go test ./...
• Dagger target discovery across all existing metadata files
• Repository .github/workflows/test.yml unit-test job via act
• Docker Buildx --check for pg-crash, pg-ivm, pgaudit, pgrouting, pgvector, postgis, timescaledb-oss, and wal2json