Add unit tests for environment creation step-flows (venv/conda) #1623

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.