Add unit tests for environment creation step-flows (venv/conda)

**Environment creation is the single most important path in the extension, but the step-based creation flows have **zero direct unit tests**. Today `src/managers/builtin/venvStepBasedFlow.ts` and `src/managers/conda/condaStepBasedFlow.ts` are only exercised indirectly through the slow, host-based integration suite (`src/test/integration/envCreation.integration.test.ts`), which means regressions in the multi-step logic (option gathering, quick-create vs. guided, cancellation/back navigation) can slip through fast unit runs.**