test(stack): stabilize postgres persistence e2e

[7ttp]

TL;DR

Stabilize the Postgres persistence e2e by checking only the Docker containers created during native stack startup.

The test previously checked every matching container on the runner. A container left by another test could therefore cause a failure even when native startup used no Docker containers.
Capture matching container IDs before startup and assert that no new IDs appear afterward.
This follows the stack test suite's existing snapshot and diff pattern while preserving Docker startup and data persistence coverage...

ref:

spotted on: https://github.com/supabase/cli/actions/runs/31634963213/job/94242831412

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