Support runtime provider, model, and command registration from extensions

## Summary

A user reported that extensions can register providers, models, and commands in the live session registry, but the Copilot app does not fully consume those updates.

## Reported behavior

An extension can:

- Register a provider and model through `session.rpc.provider.add`
- Register slash commands
- Emit `commands.changed`

This works in Copilot CLI. In the app:

1. The model picker does not refresh after registration.
2. If the model later appears, selecting it can fail because the app treats it as a persisted provider.
3. Extension commands do not appear after `commands.changed`.

## Expected behavior

The app should consume the live session registry, refresh models and commands after registration, and select extension models by their provider-qualified ID.

Start by tracing how the app consumes the live session registry after session.rpc.provider.add and commands.changed. Follow the model picker and extension-command refresh paths, then verify that newly registered models and commands appear and that model selection uses the provider-qualified ID.

Written by the indexing model from the issue text.