Openai-responses

Adds routes for models and retrieving file content.

Closed:

- [22: feat: support models endpoint](https://github.com/mharrisb1/openai-responses-python/issues/22)
- [46: feat(routes): add retrieve file content route](https://github.com/mharrisb1/openai-responses-python/issues/46)

Adds setter for `OpenAIMock` state store

> [!IMPORTANT]
> Breaking change

> [!NOTE]
> ✨ Streaming support is here

Tons of changes:

- New `EventStream` and `AsyncEventStream` objects to create mock event streams
- `OpenAIMock` class now exposes state store through `state` property
- Updated and more organized API
- Replacement of `calls` property on routes in favor of `route` property
- New examples:
- Create run with streaming
- Create run with streaming (async)
- Exporting router as transport to use as `http_client` in OpenAI client constructor
- Plus some small bug fixes and QoL enhancements

Overriding base URL was not working properly for Azure endpoints. Thanks mapohjola for pointing this out. This moves the version prefix (i.e. `/v1`) from the OpenAI routes to the default base URL. Also added an example using Azure endpoints.

Closed:

- [9: feat: base url override](https://github.com/mharrisb1/openai-responses-python/issues/9)

Fixes incorrect partial type definition for run step tool calls.

Adds `router` property on `OpenAIMock` class to expose instance of [respx.MockRouter](https://github.com/lundberg/respx/blob/366dd0bea824464e6ec9242a88f9b390a9dd74cb/respx/router.py#L323) to easily allow user to add additional routes to mock like non-OpenAI API calls, or enable a call to a route to pass through to the external service.

Usage example can be found [here](https://github.com/mharrisb1/openai-responses-python/blob/main/examples/test_router_usage.py).

Closed:

- [36: feat(api): expose RESPX router as property](https://github.com/mharrisb1/openai-responses-python/issues/36)