feat(llm): add vision, streaming, and embedding support
- ContentPart types (TextContentPart, ImageUrlContentPart) for multipart messages
- ChatMessage.content now accepts string | ContentPart[] for vision
- EmbeddingRequest/Response types + optional embed() on LLMProvider
- chatCompletionStream() implemented in OpenAI + Azure providers (SSE parsing)
- embed() implemented in OpenAI + Azure providers
- Vision helpers: isVisionMessage, hasVisionContent, buildVisionMessage, getMessageText
- MockLLMProvider: streaming, embedding, vision content support
- 27 tests passing (up from 7)