Add required 'begin' config for testing microbatch models

[MichelleArk]

resolves #315
docs dbt-labs/docs.getdbt.com/#

Problem

dbt-core is adding a required config, named begin for microbatch models in dbt-labs/dbt-core#10756.

Solution

Add required begin config so tests continue to pass once the above dbt-core PR is in.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• 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

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[MichelleArk]

Postgres testing:
✅ Using this branch + dbt-core@main (temp state until dbt-core PR is merged): https://github.com/dbt-labs/dbt-postgres/actions/runs/10999889426

✅ Using this branch + dbt-core@microbatch-begin-config (eventual state): https://github.com/dbt-labs/dbt-postgres/actions/runs/10999672904