OpenAISpec rejects standard required and named function tool_choice values

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## 🐛 Bug

`OpenAISpec` rejects two standard Chat Completions `tool_choice` values during request validation:

- `"required"`
- a named function choice such as `{"type": "function", "function": {"name": "lookup_weather"}}`

The current request model accepts only `"auto"`, `"none"`, and the non-standard compatibility value `"any"`. As a result, clients using either standard form receive HTTP 422 before `LitAPI.decode_request` runs. OpenAI's current [Chat Completions API reference](https://developers.openai.com/api/reference/cli/resources/chat) documents both `"required"` and a named function object in `tool_choice`.

### Reproduction

On current `main` (`7675990`):

```python
from litserve.specs.openai import ChatCompletionRequest

base = {
"model": "lit",
"messages": [{"role": "user", "content": "Use the weather tool"}],
}

ChatCompletionRequest(**base, tool_choice="required")
ChatCompletionRequest(
**base,
tool_choice={"type": "function", "function": {"name": "lookup_weather"}},
)
```

Both constructions raise a Pydantic validation error saying the input should be `auto`, `none`, or `any`. The same payloads sent to `/v1/chat/completions` return HTTP 422.

### Expected behavior

`OpenAISpec` should accept the standard `"required"` literal and named-function object, preserve them on `ChatCompletionRequest`, and pass them through the existing request/context path. Existing `"auto"`, `"none"`, and `"any"` behavior should remain compatible.

### Proposed scope

Extend only the `tool_choice` request schema and add focused CPU-only request-model and endpoint regressions. This does not change tool execution or selection inside user code.

### Environment

- LitServe `main` at `7675990`
- Python 3.12
- CPU only; no model download, GPU, or hosted service required

I used Codex assistance to investigate and prepare this report. No maintainer agreement or human review is claimed.

Start with ChatCompletionRequest in litserve.specs.openai and trace the /v1/chat/completions request path through LitAPI.decode_request. Add focused CPU-only request-model and endpoint regressions for required and named function tool_choice values, while confirming auto, none, and any remain compatible.

Written by the indexing model from the issue text.