providers/cli: constrain cogitate provider environments
Build cogitate CLI environments from provider-specific allowlists instead of copying the full parent environment, and require usable Vertex service-account credentials before spawning Gemini CLI. Provider call sites now pass provider names directly and tests cover cross-provider leakage, Vertex strict validation, and settings-file behavior.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>