Add support for openai-compatible APIs that do not support streaming

[jackmpcollins]

Currently OpenaiChatModel has stream=True and parses streamed responses. For some openai-compatible APIs streaming is not supported, which means they cannot be used with magentic.

For example ollama does not support streaming: #207 Also openai's o1-preview does not support streaming (or function calling) currently, so cannot be used in magentic even just to generate string responses.

Implementation could probably be just converting a non-streamed response into the streamed format (with just one chunk, or one chunk per content/tool call section), and then reuse the existing code.

[mnicstruwig]

Ah nice -- ran into this issue while implementing a custom copilot for SambaNova: OpenBB-finance/copilot-for-openbb#28

We had to abandon magentic's chat models for this reason (but still borrowed the function schema machinery).