fix(runtimes): harness setup reuses an installed uv

[faresobeid]

What

_ENSURE_UV (runtimes/base.py; also used by mcp/launch.py) now runs command -v uv first and skips both pip install -q -U --user uv and the astral installer when uv is already on PATH. The PATH export still comes first, so a ~/.local/bin/uv from an earlier rollout on the same box counts. Images without uv take the same install path as before.

Why

In a pipeline opening ~90 concurrent rollouts on prime sandboxes, about 3% of opens failed in harness setup because _ENSURE_UV downloads uv on every prepare_uv_script (pip on PyPI, else astral.sh) over flaky egress — even on images that ship uv. A uv that is already present needs no network at all.

Behavior change: uv is no longer upgraded to the latest release when one is present. prepare_uv_script needs a uv that knows uv sync --script / uv python find --script; an image with a very old pre-installed uv would now surface uv's own argument error instead of being silently upgraded.

Validation

• tests/v1/test_runtimes.py runs the real snippet under sh with a stub pip (leaves a marker, drops a uv into ~/.local/bin) and an optional stub uv: with uv present, pip does not run; without it, pip runs and uv is then found. Also asserts command -v uv precedes pip install in the composed command.
• e2e test_single_turn[null-harness-in-subprocess] and test_agentic[bash-harness-in-subprocess] pass (they run prepare_uv_script on a host with uv pre-installed).
• uv run pytest tests/v1 -m "not e2e", ruff, pre-commit pass.

Note

Fix _ENSURE_UV to reuse an installed uv on PATH

• Changes the shell sequence in base.py to check whether uv is already discoverable before the pip-install branch; the curl/wget fallback still follows.
• Adds test helpers in test_runtimes.py that stub pip and uv on a temporary PATH, plus a test asserting existing uv is preserved and missing uv triggers installation.
• Behavioral Change: runtime setup now skips installation when uv is already on PATH, instead of always running the pip branch.

^{Macroscope summarized b94df19.}