[Voltagent] Add `andChat` workflow step for multi-turn chat

### Is your feature request related to a problem? Please describe. Workflows solve a lot of problems, but today there is no clean way to have **multi-turn chat** within a workflow step. Use cases include: - An **interview step** that gathers details about a work item (multiple back-and-forth questions and answers). - A **guided process** where a step prompts the user, processes that input, then produces a document or outcome—with transitions that are not always black-and-white. Currently: - **`andAgent`** is single-turn: it uses `generateObject` under the hood, returns structured output when complete, and does **not** support streaming or tools ([docs](https://voltagent.dev/docs/workflows/steps/and-agent/)). So it cannot drive a real multi-turn conversation inside a workflow. - The **Vercel AI SDK** integration ([docs](https://voltagent.dev/docs/ui/ai-sdk-integration/)) and **`/agents/:id/chat`** endpoint work with **agents only**, not with workflows. `useChat` is built for agent