v1.1.48-open-runtime-fork.3

feat: expose RuntimeAPI to plugins for LLM completion calls

Adds a new RuntimeAPI that plugins can use to make LLM completion calls
without disrupting the active session. The API provides:
- stream(): Stream LLM completions with deltas
- getProviders(): List available providers
- getModels(): List available models for a provider

This allows the Dart runtime plugin to make LLM calls through the
official plugin API rather than relying on internal module imports.