Export MultiProvider in public API (Fix #1325)

[kobol]

Description

Resolves #1325

This PR addresses the API inconsistency where MultiProvider is used as the default factory for RunConfig.model_provider but is not exported in the public API. The changes include:

1. Export MultiProvider directly in __init__.py
2. Maintain backward compatibility while providing more flexibility

Changes Made

1. Export MultiProvider in __init__.py

# Added import
from .models.multi_provider import MultiProvider

# Added to __all__ list
__all__ = [
    # ... existing exports
    "MultiProvider",
    # ...
]

Usage Examples

Before (not possible):

from agents import MultiProvider  # This would fail

After:

# Direct access
from agents import MultiProvider, RunConfig
config = RunConfig(model_provider=MultiProvider(openai_api_key="your-key"))

## Testing

- [x] Verified that MultiProvider can be imported directly
- [x] Verified that existing functionality is not broken

## Related Issue

Fixes #[1325] - MultiProvider not exported in public API but used as default in RunConfig