Update BATS diagnostics Python stubs

[codeforester]

Summary

• Teach the BATS fake Python implementations the diagnostics capability probe used by production setup checks.
• Preserve the existing behavior for the other fake-Python command forms.

Issue

Closes #1772

Validation

• Focused project-artifact check: 1 passed.
• Focused setup-common delegation test: 1 passed.
• bash -n and shellcheck --severity=error passed for setup_helpers.bash.

Demo Impact

None.

Notes

• .ai-context/ is unchanged because this is a narrow test-fixture compatibility fix.
• The broader local macOS check/ci/doctor BATS run still has pre-existing JSON-diagnostic fixture failures beyond this capability-probe fix; the focused regression that originally failed now passes.

Checklist

• Tests added or updated
• No unrelated changes