Update the default inputs on the integration tests workflow to match the branch

[mikealfare]

Problem

We're defaulting to main in integration tests for the adapter and core branches. The scheduled release branch tests don't pass inputs, so they check out 1.9.latest and then that workflow in turn checks out the default, main. This does not affect prior release branches since the functionality to select a branch was introduced after we cut the 1.8.latest branch.

Solution

• update the defaults for dbt-core and dbt-bigquery to be 1.9.latest
• handle future release branch tests in the monorepo

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 dbt-bigquery contributing guide.

[mikealfare]

Here's a manual run of this workflow: https://github.com/dbt-labs/dbt-bigquery/actions/runs/12308203089