Add context overflow detection utilities

Extract overflow detection logic into reusable utilities:
- isContextOverflowError() to detect overflow from error messages
- isContextOverflowFromUsage() to detect overflow from token usage
- Patterns for Anthropic, OpenAI, Google, xAI, Groq, OpenRouter, llama.cpp, LM Studio

Fixes #129

packages/ai/src/index.ts

@@ -6,4 +6,5 @@ export * from "./providers/openai-completions.js";
 export * from "./providers/openai-responses.js";
 export * from "./stream.js";
 export * from "./types.js";
+export * from "./utils/overflow.js";
 export * from "./utils/typebox-helpers.js";