ci: run pytest via uv run to fix venv PATH breakage from #9214

[lstein]

Summary

PR #9214 switched the python-tests workflow's install step from uv pip install --editable ".[test]" to uv sync --no-progress --locked --extra test. This broke every pytest matrix job with:

pytest: command not found
##[error]Process completed with exit code 127.

(Visible on PR #9246's CI; only one job shows the error — the rest report "canceled" because the matrix's default fail-fast kills them once the first fails.)

Root cause

The old uv pip install honored UV_SYSTEM_PYTHON: 1 and installed into the runner's system Python, putting pytest on the PATH. uv sync is a project command — it always installs into a project-local .venv and ignores UV_SYSTEM_PYTHON — so the bare pytest invocation in the next step never sees the synced environment.

Fix

• Run pytest via uv run --no-sync pytest. The --no-sync flag matters: a plain uv run pytest would first re-sync the environment without --extra test, uninstalling pytest right before trying to run it.
• Drop UV_SYSTEM_PYTHON: 1 — dead config now that nothing in the job uses the uv pip interface.
• Drop the actions/setup-python step — setup-uv is already given the matrix python-version, and uv sync provisions its own interpreter.

Related Issues / Discussions

• Follow-up to ci: use uv.lock when running tests #9214 (ci: use uv.lock when running tests)
• Failure visible on fix(lora): sidecar-patch fp8 weights to avoid float8 add crash #9246's CI runs

QA Instructions

Note that this PR's own pull_request CI run skips the pytest steps (no python files changed, and the changed-files filter only matches pyproject.toml/invokeai/**/tests/**), so a green check there is vacuous. To exercise the fix for real, a workflow_dispatch run with always_run=true was triggered on this branch: https://github.com/invoke-ai/InvokeAI/actions/runs/27374428547 — all six matrix jobs should reach pytest and pass.

Merge Plan

Straight merge. Unblocks CI for all open PRs touching python files.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) — n/a, CI config only
• ❗Changes to a redux slice have a corresponding migration — n/a
• Documentation added / updated (if applicable) — n/a

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