Swagger UI does not expose request parameters for /predict, causing 500 error when invoked

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The `/predict` endpoint in the example servers does not define a request body schema. Consequently, Swagger UI provides no input form and sends an empty request body by default when "Execute" is clicked. This results in a 500 Internal Server Error as the server fails to parse the empty JSON payload.

## To Reproduce

### 1. Start the server

```bash
python tests/simple_server_diff_port.py
```

### 2. Access Swagger UI

Navigate to:

```
http://localhost:8000/docs
```

### 3. Trigger the Error

- Expand the `/predict` endpoint
- Click **Execute**

**Note:** No input field is visible to the user.

## Actual Results

The server crashes with a JSONDecodeError because Swagger sends an empty body:

```
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
...
File "src/litserve/server.py", line X, in predict
return await request.json()
```

## Expected Behavior

Swagger UI should display a request body form (e.g., a JSON blob like below). This allows users to provide input values directly in the interface, ensuring the `request.json()` call has valid data to parse.

```json
{
"input": 5
}
```

## Suggested Fix: Implement Pydantic Schemas

To resolve this, define a Pydantic model for the request payload. This allows the underlying FastAPI engine to generate the proper OpenAPI documentation for Swagger.

### Proposed change

```python
from pydantic import BaseModel
from typing import Any

# Define the expected schema
class PredictRequest(BaseModel):
input: Any

# Update the endpoint logic to reference this schema
# so Swagger generates the following interactive form:
# {
# "input": "value"
# }
```

## Environment

- OS: Linux
- Python: 3.12
- Installation: pip

Start with tests/simple_server_diff_port.py and inspect how the /predict endpoint is declared and how its request body is parsed. Run the server, open /docs, and verify that Swagger UI exposes a JSON input form for /predict. Done means a request such as {"input": 5} can be submitted without the empty-body JSONDecodeError or a 500 response.

Written by the indexing model from the issue text.