feat: add Snowflake privatelink host injection and fix BigQuery preflight check

[MiroCillik]

AJDA-2149

Follow up to #132

Summary

• Inject Snowflake host parameter into profiles.yml only for privatelink connections, where the hostname differs from what dbt infers from the account name
• Fix BigQuery dataset preflight check: use dataset->update() (write operation) instead of dataset->reload() (read-only) to properly verify write permissions before dbt runs CREATE SCHEMA IF NOT EXISTS
• Widen DbtProfilesYaml::dumpYaml() to accept additionalOptions injected into every output after merge

Test plan

• Unit tests pass for DbtYamlCreateTest (profiles with host for privatelink URLs)
• Unit tests pass for LocalBigQueryProviderTest (dataset write permission check)
• CI tests-in-kbc passes (BigQuery jobs no longer fail with 403)
• Verify Snowflake privatelink connections work with injected host in profiles.yml

Release Notes

• Justification

  • Snowflake Private Link connections fail because dbt derives the hostname from the account name, which can produce incorrect hostnames (e.g. when the host contains underscores). The host parameter override is needed only for privatelink — regular Snowflake connections work fine with just the account.
  • BigQuery preflight check was using a read-only API call (datasets.get) to verify eventual consistency of IAM permissions, but dbt needs write permission (datasets.create). GCP IAM propagates read permissions faster than write, so the check passed but dbt still failed with 403.

• Plans for Customer Communication

  • No customer communication needed. Host injection is automatic for privatelink URLs. BigQuery fix is transparent.

• Impact Analysis

  • Low risk. Host is only injected when the Snowflake URL contains "privatelink" — no change for regular connections. BigQuery fix changes the preflight check from a read to a no-op write, matching what dbt actually needs.

• Deployment Plan

  • Continuous deployment. No special rollout needed.

• Rollback Plan

  • Fully reversible. Revert/redeploy previous version.

• Post-Release Support Plan

  • No post-release support needed.

🤖 Generated with Claude Code

[MiroCillik]

Tohle je jeste fix predesleho PR na retry u BQ. Ten "reload" totiz overiril jen "read" a ty "write" permissions se propaguji pomaleji, takze ten retry pak nefungoval spravne.