Fix is_api_message to correctly exclude reasoning messages

[448523760]

Problem

The is_api_message function in conversation_history.rs had a misalignment between its documentation and implementation:

• Comment stated: "Anything that is not a system message or 'reasoning' message is considered an API message"
• Code behavior: Was returning true for ResponseItem::Reasoning, meaning reasoning messages were incorrectly treated as API messages

This inconsistency could lead to reasoning messages being persisted in conversation history when they should be filtered out.

Root Cause

Investigation revealed that reasoning messages are explicitly excluded throughout the codebase:

1. Chat completions API (lines 267-272 in chat_completions.rs) omits reasoning from conversation history:

   ResponseItem::Reasoning { .. } | ResponseItem::Other => {
       // Omit these items from the conversation history.
       continue;
   }

2. Existing tests like drops_reasoning_when_last_role_is_user and ignores_reasoning_before_last_user validate that reasoning should be excluded from API payloads

Solution

Fixed the is_api_message function to align with its documentation and the rest of the codebase:

// Before: Reasoning was incorrectly returning true
ResponseItem::Reasoning { .. } | ResponseItem::WebSearchCall { .. } => true,

// After: Reasoning correctly returns false  
ResponseItem::WebSearchCall { .. } => true,
ResponseItem::Reasoning { .. } | ResponseItem::Other => false,

Testing

• Enhanced existing test to verify reasoning messages are properly filtered out
• All 264 core tests pass, including 8 chat completions tests that validate reasoning behavior
• No regressions introduced

This ensures reasoning messages are consistently excluded from API message processing across the entire codebase.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[448523760]

I have read the CLA Document and I hereby sign the CLA

[448523760]

recheck

[448523760]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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[448523760]

Hi @jif-oai, could you please take a look at this PR when you have time?

[448523760]

rebased onto main