Run Astra integration tests in merge queue only #418

Run the Astra integration tests only in the merge queue, not on every PR push.

These tests (tests/base/integration) all hit one shared Astra DB, so concurrent PR runs collide with each other: 409 conflicts, and false failures from the empty-database guard added in #423. The merge queue runs one check at a time, right before merge, which removes the collisions while still gating what lands.

Change

• Run the Astra integration job on merge_group only.
• Keep unit tests and the local Data API integration job (its own isolated DB) on every PR push.
• Require the merge-queue Astra run to pass before merging.

Done when

• Astra tests no longer run on PR pushes and never run concurrently.
• Merging still requires a passing Astra run.
• Per-PR checks are unchanged.

Note: this also needs merge queue enabled on main in the repo settings.