feat: migrate SambaNova provider to AI SDK

- Replace BaseOpenAiCompatibleProvider with BaseProvider + AI SDK
- Use dedicated sambanova-ai-provider package
- Implement createMessage() using streamText with streaming support
- Implement completePrompt() using generateText
- Add processUsageMetrics() for cache token handling from providerMetadata
- Add getMaxOutputTokens() and getLanguageModel() helpers
- Use shared AI SDK utilities (convertToAiSdkMessages, handleAiSdkError, etc.)
- Set default temperature to 0.5 following Fireworks pattern
- Update tests to match AI SDK streaming behavior with async generators
- Add comprehensive tests for usage metrics, temperature handling, tool handling, and error handling

Author: roomote