fix: use extra_body for DeepSeek V3.1 Terminus reasoning control

[roomote]

Per request to create a new PR that only adds extra_body for DeepSeek V3.1 Terminus (no integration test changes). Implementation: in OpenRouterHandler, add extra_body with chat_template_kwargs.thinking for DeepSeek-V3.1-Terminus and avoid passing the OpenRouter reasoning param for this model. Relevant unit tests pass locally.

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Important

Add extra_body for DeepSeek V3.1 Terminus in OpenRouterHandler to control reasoning, replacing reasoning param for this model.

• Behavior:
  • Add extra_body parameter in OpenRouterHandler for DeepSeek V3.1 Terminus to control reasoning.
  • extra_body uses chat_template_kwargs.thinking based on reasoning settings.
  • Skip reasoning param for DeepSeek V3.1 Terminus; use extra_body instead.
• Implementation:
  • Modify createMessage and completePrompt methods in openrouter.ts to include extra_body for DeepSeek V3.1 Terminus.
  • Check modelId for "DeepSeek-V3.1-Terminus" to conditionally apply extra_body.

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[roomote]

Review Summary

I've reviewed the changes in this PR. Found 2 code quality improvements:

• Remove unnecessary type assertion at line 151 - use reasoning?.exclude instead of (reasoning as any)?.exclude
• Remove unnecessary type assertion at line 270 - use reasoning?.exclude instead of (reasoning as any)?.exclude

The implementation correctly adds extra_body support for DeepSeek V3.1 Terminus and properly conditionally includes the OpenRouter reasoning parameter. The logic appears sound, but the type assertions can be simplified since OpenRouterReasoningParams already defines the exclude property.

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[roomote]

The type assertion (reasoning as any)?.exclude is unnecessary. Since reasoning is of type OpenRouterReasoningParams | undefined and OpenRouterReasoningParams already has exclude?: boolean as a property, you can safely use reasoning?.exclude instead. Removing the as any cast improves type safety and maintainability.

Suggested change

	thinking: Boolean(reasoning) && !(reasoning as any)?.exclude,
	thinking: Boolean(reasoning) && !reasoning?.exclude,

[roomote]

The type assertion (reasoning as any)?.exclude is unnecessary here as well. Since reasoning is of type OpenRouterReasoningParams | undefined and OpenRouterReasoningParams already has exclude?: boolean as a property, you can safely use reasoning?.exclude instead.

Suggested change

	thinking: Boolean(reasoning) && !(reasoning as any)?.exclude,
	thinking: Boolean(reasoning) && !reasoning?.exclude,