70+ AI Providers Under Same Rust Interface

This brings the Vercel AI SDK ergonomics into Rust with a type-safe LanguageModelRequest builder, #[tool] macros to expose callable tools, streaming text and structured JSON outputs, and compatibility with Vercel UI stacks. The sheer provider count (70+) and ready-made agent tooling are compelling for Rust shops; quality will hinge on per-provider coverage and runtime compatibility, but the docs, examples, and CI indicate serious follow-through.