ci: pin ruff to 0.15.0 to avoid 0.16.0 default-ruleset breakage

ruff 0.16.0 expanded its default rule set from ~59 to ~413 rules,
causing the unpinned `uvx ruff check` step to report ~1475 pre-existing
violations unrelated to this change. Pin to 0.15.0 to restore green lint.

Assisted-by: GitHub Copilot (model: Claude Opus 4.8, autonomous)

Author: mnriem

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
           python-version: "3.14"
 
       - name: Run ruff check
-        run: uvx ruff check src tests
+        run: uvx ruff@0.15.0 check src tests
 
   pytest:
     runs-on: ${{ matrix.os }}