fix: use correct uv command

[alessio-locatelli]

https://docs.astral.sh/uv/concepts/projects/dependencies/#dependency-groups

This used to fail with

STEP 7/11: RUN uv sync --extra test-server --no-dev
Using CPython 3.13.9 interpreter at: /usr/local/bin/python3
Creating virtual environment at: .venv
Resolved 119 packages in 0.83ms
error: Extra `test-server` is not defined in the project's `optional-dependencies` table
Error: building at STEP "RUN uv sync --extra test-server --no-dev": while running runtime: exit status 2
ERROR:podman_compose:Build command failed
Error: executing /usr/sbin/podman-compose up httpbin-custom: exit status 2

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.01%. Comparing base (8d28ac7) to head (e2181ea).
⚠️ Report is 47 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   96.20%   96.01%   -0.19%     
==========================================
  Files          10       10              
  Lines        1081     1105      +24     
  Branches      188      113      -75     
==========================================
+ Hits         1040     1061      +21     
- Misses         30       31       +1     
- Partials       11       13       +2     

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