feat: graceful apple context window error handling

[JKobrynski]

Related issue - fixes #125

Summary

Adds explicit handling for Apple Foundation Models context window overflow errors.

When LanguageModelSession fails with exceededContextWindowSize, the Apple provider now exposes a stable CONTEXT_WINDOW_EXCEEDED error code. This allows apps to detect the failure and decide how to recover, for example by starting a new conversation or trimming previous messages.

Changes

• Map Apple’s native context window overflow error to CONTEXT_WINDOW_EXCEEDED
• Forward the error code through streaming errors
• Expose AppleLLMErrorCodes.ContextWindowExceeded
• Update the Expo example so text streams surface the error properly
• Document recommended app-level recovery strategies

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[Copilot]

Pull request overview

Adds explicit handling for Apple Foundation Models context-window overflow by introducing a stable CONTEXT_WINDOW_EXCEEDED error code and propagating it through streaming APIs, alongside documentation and example updates.

Changes:

• Introduces AppleLLMErrorCodes.ContextWindowExceeded and a JS error wrapper that carries an optional code.
• Forwards native error code through stream error events and converts stream errors into Error instances with code.
• Updates iOS native layer to detect exceededContextWindowSize, emit the stable error code, and documents recovery strategies (plus updates the Expo example to surface stream errors).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
website/src/docs/apple/generating.md	Documents context window behavior, stable error code, and recommended recovery patterns (including streaming guidance).
packages/apple-llm/src/stream.ts	Wraps stream error events using createAppleLLMError(..., code) to preserve an error code.
packages/apple-llm/src/NativeAppleLLM.ts	Extends stream error event shape with optional code.
packages/apple-llm/src/index.ts	Exports new Apple error types and constants.
packages/apple-llm/src/errors.ts	Adds AppleLLMErrorCodes and createAppleLLMError helper for code-carrying errors.
packages/apple-llm/src/ai-sdk.ts	Forwards stream error code into Error objects for AI SDK streaming consumers.
packages/apple-llm/ios/AppleLLMImpl.swift	Adds native mapping for context-window overflow errors and forwards code through streaming callbacks.
packages/apple-llm/ios/AppleLLMError.swift	Adds contextWindowExceeded case and stable CONTEXT_WINDOW_EXCEEDED code mapping.
packages/apple-llm/ios/AppleLLM.mm	Emits stream error payloads with optional code field.
apps/expo-example/src/screens/ChatScreen/index.tsx	Captures streamText errors via onError so text streaming surfaces failures properly.

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[artus9033]

LGTM from my end after:

1. let's double-check if the above Copilot comment is a false-positive
2. this PR is missing a convenience feature - exposing a JS API that would allow users to count the number of tokens a string takes, before running inference on the model (+ docs update for that); let's address this here, or in a follow-up PR