fix: harden connectors and CLI diagnostics

[skadel]

Summary

• resolve dbt models through compiled SQL using external configuration
• keep Snowflake and cache-only dialects off the BigQuery fallback
• provide actionable, structured Vertex access diagnostics
• document connector behaviour and add regression coverage

Validation

• ruff check .
• ruff format --check .
• targeted tests passed (15)
• mocksql --help and non-interactive init smoke tests passed

Depends on #45 for the 0.2.1 distribution changes.