Fix OpenAI Provider with GitHub Models

[rzhade3]

Goose was failing to properly serialize streamed content from GitHub Models using the OpenAI provider due to subtle differences in the response body from OpenAI vs GitHub Models. Namely:

1. The first streamed chunk from GitHub Models does not contain a model key.
2. The final chunk from GitHub Models, which contains the usage information, has content: null, which needs to be special cased as this is neither a text block nor a tool block.

I have addressed these issues by making the streaming parser logic less strict, allowing for slight variations in implementation to allow GitHub Models to continue to be usable with Goose.

I had originally reported this in #3615, but mistakenly closed it out as a duplicate of an unrelated issue.

If you'd like to diff the OpenAI vs GitHub Models response bodies, I've provided them below:

OpenAI vs GitHub Models API Sample Responses

OpenAI Response

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data: [DONE]

GitHub models response

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[michaelneale]

very nice catch - need to triple check still works with openai. It is a challenge that so-called openai compatible endpoints never seem to be, cc @DOsinga if you think we have sufficient openai coverage for this (if not perhaps need to have streaming test cases added for this PR?)

[DOsinga]

Nice find! thanks for jumping on to this