Fix BigQuery source alias deprecation warning (#12327)

[alexaustin007]

Resolves #12327

Problem

When users define BigQuery sources with dataset/project at the top level (valid aliases for schema/database), dbt-core emits a false PropertyMovedToConfigDeprecation warning stating the property should be moved into config. This regression appeared after #12291 introduced alias-aware JSON schema validation.

Solution

Add a scoped guard in JSON schema validation to treat dataset and project as valid top-level source aliases when the active adapter includes BigQuery, preventing the incorrect deprecation warning. The guard only applies to sources[] paths, so other objects and adapters continue to warn appropriately. Added unit coverage for BigQuery source alias cases and a non‑BigQuery control.

Checklist

• [ X] I have read the contributing guide and understand what's expected of me.
• [ X] I have run this code in development, and it appears to resolve the stated issue.
• [X ] This PR includes tests, or tests are not required or relevant for this PR.
• [X ] This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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