feat(core): add Python group to TEST_PATTERN_GROUPS with pytest patterns

Introduce a Python entry in TEST_PATTERN_GROUPS with the two dominant
discovery conventions:

  **/test_*.py   — pytest / unittest default (Django, Flask, requests, …)
  **/*_test.py   — Google/TensorFlow style (tensorflow, jax, some Meta libs)

Python testing tools rely on filename-based discovery, so large codebases
routinely co-locate test files with the module under test rather than
funnelling everything into a single tests/ tree — mirroring the C++
rationale for file-pattern rules introduced in #480.

Tests: assert Python sub-header + both pattern emissions and extend the
stable-order invariant to place Python after C++.