feat(core): Keep track of test case executions during test run (no-changelog)

[burivuhster]

Summary

This PR introduces a new entity "TestCaseExecution". It represents a specific past execution being "re-run" and evaluated during a specific Test Run.

The idea is to store the connection between Test Runs and related executions (past, new and evaluation) in a separate table, together with statuses, errors, individual metrics produced by the evaluation workflow. This will allow us to keep track of the details of each Test Run even after related executions will be pruned, and make data fetching more efficient and straightforward in general. Previously, we had to rely on a workaround with the testRunId property added to each evaluation execution's metadata.

Related Linear tickets, Github issues, and Community forum posts

• https://linear.app/n8n/issue/AI-524/link-evaluation-executions-to-test-runs-and-test-cases

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)