Fix custom database image on initial IaC apply

[SandeepVaman]

Summary

• reassert a database's authored source in the same change set that creates it
• prevent a custom database image from being replaced by the current engine default on first apply
• add an offline regression test covering a PostgreSQL 17 image pin

Fixes railwayapp/railway-ts-sdk#81

Why this is in the CLI

The TypeScript SDK already emits the requested image in both database.image and database.source.image. The observed divergence happens when the initial resource.create change is applied. Subsequent resource.update changes already reconcile the source correctly, so this patch makes that source update explicit during database creation.

Testing

• git diff --check
• cargo test iac::tests::reasserts_a_custom_database_image_during_creation (Rust 1.90 container; 1 passed)

Integration validation

Please validate against the server-side change-set executor that a resource.update for the newly created database address is ordered after resource.create and that the first deployment starts with the authored image.

Maintainer note: external contributors cannot apply repository labels; please add release/patch for the required label check.