Parallelize slow integration checks with a dynamic matrix

[tkilias]

Summary

Parallelize the slow integration checks using a generated workflow matrix.

Problem

The slow integration checks currently run as a single unit, which makes CI slower and reduces visibility into which specific slow test target failed.

Proposed change

• add a dedicated matrix workflow for slow checks
• generate the matrix from noxfile.py
• run the slow-check job against that matrix so the test targets execute independently

Impact

This should reduce wall-clock time for slow checks and make failures easier to isolate in CI.

Related PR

• [codex] Parallelize slow checks #169