Make strict QA Aqua subprocess self-contained

[ChrisRackauckas-Claude]

Summary

• preserve QA environments that already declare Aqua
• create an isolated manifest-backed Aqua environment only for minimal QA environments
• cover Aqua's ambiguity subprocess behavior with a regression test
• retain the global Julia broadcast extension protocol exception

Verification

• SciMLTesting full test suite on Julia 1.12
• downstream OrdinaryDiffEqOperatorSplitting strict QA: all shared checks pass; the only remaining failure is the unreleased OrdinaryDiffEqCore.failfactor_default public API

Ignore until reviewed by @ChrisRackauckas.