feat: integrate ai-sdk provider

[uinstinct]

Description

Adds a separate ai-sdk provider into openai-adapters for usage in the cli.

AI Code Review

• Team members only: AI review runs automatically when PR is opened or marked ready for review
• Team members can also trigger a review by commenting @continue-review

Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

Screen recording or screenshot

[ When applicable, please include a short screen recording or screenshot - this makes it much easier for us as contributors to review and understand your changes. See this PR as a good example. ]

Tests

[ What tests were added or updated to ensure the changes work as expected? ]

---

Summary by cubic

Add a new ai-sdk provider so the CLI can use Vercel AI SDK models (OpenAI and Anthropic) for chat and embeddings. Also convert assistant tool_calls to the AI SDK format to enable tool use.

• New Features

  • Added AiSdkApi with chat completions (streaming and non-streaming) and embeddings via @ai-sdk/openai and @ai-sdk/anthropic.
  • Accepts model as "provider/model" (e.g., "openai/gpt-4o"); CLI now forwards the model field.
  • Converts assistant tool_calls to Vercel AI SDK messages; supports tools, tool_choice, and streaming back to OpenAI-style chunks.
  • Wired into constructLlmApi and types (AiSdkConfig); legacy completions, FIM, and rerank are not supported.

• Migration

  • In CLI config, set provider: "ai-sdk" and model: "/" (e.g., "openai/gpt-4o" or "anthropic/claude-3.5-sonnet").
  • Provide the usual apiKey/apiBase and any requestOptions as needed.

^{Written for commit 1689ac2. Summary will update on new commits.}