Update openai with responses api

[ericelskenpmg]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[meidad]

Thanks for this — Responses API support is genuinely useful, especially for file inputs and reasoning models. A few things before merging:

Required

1. Fill out the PR description. The body is blank and all checklist items are unchecked. Please describe what changed and how you tested it.

2. Auto-activation behavior. It looks like shouldUseResponsesAPI may auto-route reasoning models (o1/o3/o4 series) through the Responses API even without WithResponsesAPI(true). If so, that's a silent behavior change for existing reasoning model users. Please make it fully opt-in, or at minimum document the auto-activation prominently.

3. FileInputs in shared interface. Adding FileInputs to GenerateOptions in pkg/interfaces/llm.go affects all providers. Anthropic, Gemini, and DeepSeek will silently ignore these inputs. That's acceptable, but each provider's Generate/GenerateWithTools should log a warning or return an error when FileInputs is non-empty and the provider doesn't support it. Silent no-ops are hard to debug.

Nice-to-have

4. A short example in examples/ showing Responses API + file input end-to-end would be helpful for users.

Happy to merge once items 1–3 are addressed.