Fail clearly when an explicitly requested environment cannot activate

[protostatis]

Summary

activate_venv() returns False for an invalid environment, but startup ignores that result and continues. session.json can then report the requested environment even though activation failed and code is running under a different interpreter.

Also, the current if venv_path is None and not args.no_conda branch means --no-conda suppresses explicit --conda, not only automatic fallback.

Expected

• An invalid explicit --venv or --conda should fail startup.
• session.json and status should report only a successfully activated environment.
• --no-conda should disable automatic conda fallback without overriding an explicit --conda request.

Priority context

This is mainly a configuration-error path, but silent fallback is difficult to diagnose and can run code with the wrong dependencies.