feat: refactor OpenRouter provider to use Vercel AI SDK

- Replace direct OpenAI SDK usage with @openrouter/ai-sdk-provider
- Add 'ai' package for streamText() and generateText() functions
- Extract reusable AI SDK conversion utilities to src/api/transform/ai-sdk.ts:
  - convertToAiSdkMessages(): Anthropic messages to CoreMessage format
  - convertToolsForAiSdk(): OpenAI tools to AI SDK tool format
  - processAiSdkStreamPart(): AI SDK stream events to ApiStreamChunk
- Update tests to mock AI SDK functions instead of OpenAI client
- Add comprehensive tests for the new ai-sdk.ts utility module

This follows Vercel's AI SDK provider pattern for standardized LLM integration
and enables easier migration of other providers in the future.

Author: daniel-lxs