feat: migrate HuggingFace provider to use AI SDK @ai-sdk/huggingface package

- Add @ai-sdk/huggingface ^1.0.28 dependency
- Rewrite HuggingFaceHandler to extend BaseProvider with AI SDK
- Use streamText/generateText from ai package for streaming and completions
- Use shared utilities: convertToAiSdkMessages, convertToolsForAiSdk, processAiSdkStreamPart, mapToolChoice, handleAiSdkError
- Add processUsageMetrics for cache token handling from providerMetadata
- Add comprehensive test suite following Fireworks test patterns
- Default temperature: 0.5
- Base URL: https://router.huggingface.co/v1 (HuggingFace Responses API)

This follows the exact pattern from PR #11118 (Fireworks migration).

Author: roomote