ci: pin uv to 0.11.19 + graphwalks example_rollouts fix (from main)

Shared CI fixes for the martas/1434-stacked CLI work: pin uv (0.11.20 drops
pinned deps -> 7 servers fail; = #1576) and pull main's graphwalks
example_rollouts.jsonl (fixes its data validation). This branch is the base
for the ng_validate (#1599) and config-error (#1609) PRs so the fixes live in
one place. Drop when martas/1434 rebases on main.

Signed-off-by: Wojciech Prazuch <wprazuch@nvidia.com>

Author: wprazuch

.github/workflows/unit-tests.yml

@@ -134,7 +134,9 @@ jobs:
           sudo apt-get install -y --no-install-recommends git curl ca-certificates
           # The flow below should be used and synced with any Docker or container related flows. There is no script here to keep it 100% explicit.
           # This is how we test and this is how you should use/consume.
-          curl -LsSf https://astral.sh/uv/install.sh | sh
+          # Pin uv: 0.11.20 has a resolver regression that silently drops pinned deps from
+          # `uv pip install -r requirements.txt`. 0.11.19 is the latest known-good version.
+          curl -LsSf https://astral.sh/uv/0.11.19/install.sh | sh
           uv venv --python 3.12
           source .venv/bin/activate
           uv sync --extra dev