Add similar retry logic than in Slang SlangPy tests

[jkiviluoto-nv]

Once #780 and its counterpart in Slang are merged, create a followup to imitate the rertry behavior. See https://github.com/shader-slang/slang/pull/9900/changes#r2872012067

[nv-slang-bot]

Triage — retry logic for SlangPy CI tests · fix in draft PR #1123

• Status: Triaged → fix implemented, held in draft PR Add two-stage pytest retry to SlangPy CI test jobs #1123 pending human review. Dependency gate was lifted (both Support cross-repo CI testing from Slang PRs #780 merged 2026-03-02 and Decouple SlangPy CI testing via cross-repo dispatch slang#9900 merged 2026-03-03).
• Link: PR #1123 — Fixes #829 (auto-close armed; fires on merge, not while draft). Origin was the review note on slang#9900 (r2872012067) flagging SlangPy's missing retry as a pre-existing gap to fix in a separate change — this is that change.
• Verdict: enhancement / CI-test-infra · medium · P2. Fix adds run_pytest_with_retry to tools/ci.py: a failed parallel run gets one sequential rerun of only the failed tests (-n 0 --lf --lfnf=all), gated on --parallel so non-parallel sanitizer jobs never auto-retry an intermittent finding. No new dependency (--lf is core pytest; pytest-xdist already present).
• Next-action: Awaiting CI green + maintainer review on PR Add two-stage pytest retry to SlangPy CI test jobs #1123. CI so far: pre-commit + several builds + license/cla green, 0 failures, remaining platform builds still running. Held as draft — the bot will not self-ready or merge; a human maintainer opens/merges.
• Blocker: None. False-green guard against the --lf-selects-zero trap (cf. slang-test unit-test retry silently reports failed unit tests as passing in test-server mode slang#11911) is in place and positive-controlled: --cache-clear + exit-code propagation (empty selection exits 5 → red) + --lfnf=all (crash-path re-executes the real failure). 9 GPU-free regression tests cover it.

_{🤖 Generated by an automated SlangPy coworker — may be inaccurate. A human maintainer should verify.}