[DOCS] Regenerate TESTING.md — stale 'Python 3.12' (image is 3.14)

[c-vigo]

Problem

docs/templates/TESTING.md.j2:33 and generated TESTING.md:33 say 'Python 3.12, uv'; the Nix image ships CPython 3.14 (README already correct). Template not regenerated after the version bump.

Recommendation

Fix the .j2 template and run just docs.

Context

Found during the state-of-the-repo review of #670 (epic #625).

Refs: #625