model_id/provider_id sent as flat fields instead of nested model object

[raywcm]

Summary

The Python SDK sends modelID/providerID as flat top-level fields in request bodies, but the OpenCode server expects them nested inside a model object.

Affected endpoints

• session.chat() (sync and async)
• session.init() (sync and async)

Expected behavior

The SDK should nest model selection parameters:

{ "model": { "providerID": "anthropic", "modelID": "claude-opus-4-5-20251101" }, ... }

Actual behavior

{ "modelID": "claude-opus-4-5-20251101", "providerID": "anthropic", ... }

Fix

• Added ModelParam TypedDict to session_chat_params.py
• Updated SessionChatParams and SessionInitParams to use nested model: ModelParam
• Updated all 4 body constructions in session.py (sync/async chat, sync/async init)