ci(tests) Bump Python 3.14 -> 3.15

why: Run the test job on the current CPython release.

what:
- replace 3.14 with 3.15 in the `build` job matrix of tests.yml
- leave the `release` job on 3.14

The suite does not pass on 3.15 yet: `uv run py.test` reports 14
failures in tests/cli/test_formatter.py.

See also: https://devguide.python.org/versions/

Author: tony

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.14']
+        python-version: ['3.15']
     steps:
       - uses: actions/checkout@v7