fix(openaicompat): handle cerebras/glm-4.6 thinking

[Amolith]

Adds GetReasoning method to handle different providers using either reasoning or reasoning_content field names. So far, Cerebras is the only one I've noticed using this structure.

https://inference-docs.cerebras.ai/capabilities/reasoning

Assisted-by: GLM 4.6 via Crush

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

[Amolith]

At first I thought the failing tests were expected because we're now passing reasoning_content back where we weren't before, but from checking their docs, that's actually incorrect. They expect reasoning merged into the main content string instead. Is that enough of a deviation from the rest of openai-compat to warrant a new handler/provider just for Cerebras?

[kujtimiihoxha]

@Amolith to make reasoning work than for cerebras we need to handle reasoning content different so yeah we would need to create a new provider with new hooks so we include reasoning in the content.

[Amolith]

Closing this to fix it differently as a separate PR