Delete the single-implementor LanguageModel trait and merge its methods
into inherent impl on a renamed Model struct. Change provider field from
String to Provider enum, eliminating constant string↔enum conversions
across the codebase. Fix Provider serde attributes so OpenAi serializes
as "openai" (not "open_ai") to match catalog.json.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>