fix(openai-provider): only apply reasoning effort to o1/o3 models

[ZhiRuYan]

Summary

Fix OpenAI provider to only apply the reasoningEffort parameter to models that support it (o1 and o3 series). Previously, this parameter was applied to all models when OPENAI_REASONING_EFFORT was set, which could cause unnecessary overhead or potential errors with non-o1/o3 models like gpt-5.

Changes

• Added supportsReasoningEffort() helper function to check if a model supports the reasoning effort parameter
• Modified logic to only apply reasoning effort to o1/o3 models
• Updated default behavior: reasoning effort defaults to "low" only for o1/o3 models, not for all models
• Improved code comments to clarify the behavior
• Fixed variable reuse by introducing selectedModel variable for better code clarity

Motivation

The reasoning effort parameter is specific to OpenAI's o1 and o3 model series. Applying it to other models (like gpt-5, gpt-4, etc.) is unnecessary and could potentially cause issues.

Testing

• ✅ All existing tests pass
• ✅ TypeScript compilation successful
• ✅ Linting passes
• ✅ Code follows repository patterns

[dcramer]

This will break with gpt-5 which supports reasoning (and is the main reason we added this fwiw)