[Bugfix] Fix Responses API crash when ResponseReasoningItem has content=None

[rranabha]

Summary

• Fix TypeError: object of type 'NoneType' has no len() crash in response_input_to_harmony() when a ResponseReasoningItem has content=None
• Fall back to the summary field (joining all summary texts) when content is None, which occurs in multi-turn Responses API requests where previous reasoning output is passed back as input
• Add unit tests covering reasoning items with content, content=None with summary, multiple summaries, empty summary, and missing summary

Test plan

• Existing tests in tests/entrypoints/openai/responses/test_harmony_utils.py pass
• New TestResponseInputToHarmonyReasoning test class covers all content/summary combinations
• Multi-turn Responses API requests with ResponseReasoningItem(content=None) in input no longer crash

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[mergify]

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[alvinttang]

The root cause is correctly identified — content=None is a valid API state when encrypted_content is used, so the hard assert was wrong. The fallback to summary is a reasonable recovery strategy, but joining multiple summary items with a single space (e.g., "First thought. Second thought.") could produce grammatically awkward output depending on punctuation; a newline separator might be more faithful to the original reasoning structure. The remaining assert len(content) == 1 in the non-None branch is still a potential crash if the API ever returns multi-entry content arrays — worth converting to a more defensive approach (e.g., joining or taking the first item). Test coverage is thorough and the edge cases are well exercised.

[chaunceyjiang]

/cc @qandrew PTAL.

[bbrowning]

I believe this attempts to fix the same issues as #34499, but 34499 properly handles the summary field which is only meant to be user-visible summaries but not supposed to make its way back into the model's prompt.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @rranabha.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[bbrowning]

@rranabha 34499 just merged. If you're willing, please take a look at the changes merged there to ensure they fix your issue and if so we can close this PR. If you're not sure or want to chat about that, I'm happy to do that as well.