models.list API omits CLI's configured default model (e.g. claude-sonnet-4.6)

Describe the bug

The models.list JSON-RPC method returns a catalog of selectable models, but it does not include the model currently configured as the CLI's default in ~/.copilot/config.json.

For example, after running /model in the CLI and selecting claude-sonnet-4.6, the config stores "model": "claude-sonnet-4.6". However models.list returns only up to claude-sonnet-4.5 — the configured default is absent from the list.

This means any tool built on top of the SDK (e.g. the github-copilot-sdk Python package) that populates a model picker from models.list will not show the user's actual current model.

Steps to reproduce

1. Run /model in the CLI and select claude-sonnet-4.6
2. Call models.list via the SDK: models = await client.list_models()
print([m.id for m in models]) # claude-sonnet-4.6 is absent

Expected behavior

models.list should include every model the CLI can actually use — including the one it is currently configured to use as default.

Additional context

- CLI version: 0.0.414
- SDK version: github-copilot-sdk==0.1.23
- The session.start event's selected_model field is also None when a session is created without explicitly specifying a model, so there is no API workaround to discover the default model.