BI-9790: extend mcr_source_or_ref to resolve cross-schema parents on improvado_models_staging

[Solovechik]

Summary

• When target.schema == 'improvado_models_staging', the macro now queries system.tables to check if the parent model exists in any internal_analytics_dbt_staging_* schema and uses its fully-qualified name if found
• Falls back to ref() if parent is in the same schema, then source() as a final fallback
• All other environments (prod, personal staging) behave unchanged

Context

On staging, internal_analytics_src and internal_analytics models are built in internal_analytics_dbt_staging_* schemas. When a child model runs on improvado_models_staging, it needs to reference these parents by fully-qualified name since they live in a different schema.

A raw string is used because ref() and source() cannot point to arbitrary runtime schemas. This is acceptable since this code path is staging-only.

Replicated merge tree tables land in improvado_staging, but that schema is always a leaf — no models depend on it — so falling back to source() for those is correct.

Test plan

• Run a model on improvado_models_staging whose parent was built in internal_analytics_dbt_staging_* — verify it resolves to the fully-qualified name
• Run a model on improvado_models_staging whose parent is in the same schema — verify ref() is used
• Run a model on improvado_models_staging with no parent in either schema — verify source() is used
• Run on a personal staging schema — verify behavior is unchanged
• Run in prod — verify source() is always used

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