fix(agent): populate primary model from models fallback array (#121)

[warun7]

Fixes #121.

Summary

• Populated model from models[0] across callModel, resolveAsyncFunctions, and ModelResult when model is omitted.
• OpenRouter server tools (and backend request processors) require the primary model property on incoming requests.
• Synchronized model and models in buildNextTurnParamsContext and applyNextTurnParamsToRequest.
• Preserved explicit model when provided alongside models.
• Added unit tests for server tools with models fallback, explicit model preservation, and async models functions.
• Added changeset.