fix(python): provision declared deps in default tests

[AntoineToussaint]

Outcome

Formula-less Python tests now materialize the project-declared environment through the same shared derivation and uv argument builder as formula-backed tests. The default adapter still owns pytest; Mind remains blind to Python commands and package details.

The default structured runner now:

• carries requirements files, editable packaging, Python pins, groups, extras, and configured environment into uv;
• uses the existing persistent-venv path when project metadata selects it;
• classifies zero-case collection/provisioning failures as typed environment errors with the underlying diagnostic;
• keeps JUnit, caches, bytecode, and environments outside the source checkout.

Production evidence

Mind main 634780fb ran many-units-microservices-demo__service-unit-inventory__openai-deep against source 5096a85b.... Every Python unit reported formula_source: not detected, detected requirements.txt, then returned zero-case ERRORED because the old default command installed only pytest.

Verification

• go test ./runners/python -run TestRunPythonTestsStructuredMaterializesDeclaredRequirements -count=1 -v
• go test ./runners/python -count=1
• go test -race ./runners/python -count=1
• go vet ./runners/python

The new test executes real uv against a real local Python package that is reachable only through requirements.txt; it also proves the source checkout remains clean.

Fixes #164