Expose provider token usage on responses

[KotlinFactory]

Summary

• add a LanguageModelUsage type for provider-reported token usage
• expose usage on LanguageModelSession.Response and streaming snapshots
• map provider usage metadata for OpenAI, Open Responses, Anthropic, Gemini, and Ollama
• add focused tests for response and stream usage exposure

Testing

• xcrun swift build
• xcrun swift test --filter MockLanguageModelTests

Notes

• this PR is scoped to reporting token usage only
• the branch is based directly on huggingface/main

[KotlinFactory]

Background: Tracking LLM cost on a per-user or per-task basis.