[codex] fix chat completions reasoning compatibility

[KilimiaoSix]

Summary

This PR fixes an OpenAI-compatible Chat Completions bridge issue where CodeXManager can successfully proxy a /v1/chat/completions request through /v1/responses but return an apparently empty assistant message to compatible clients such as Hermes.

The visible symptom is a successful response with empty content when the upstream Responses result is reasoning-only. Hermes already understands reasoning and reasoning_content, but CodeXManager was not mapping Responses reasoning summaries back into those Chat Completions extension fields.

Root Cause

The Chat Completions compatibility path rewrites non-native OpenAI-compatible /v1/chat/completions requests to /v1/responses. The request rewrite only preserved caller-provided reasoning fields and did not request reasoning summaries by default. On the way back, the Responses-to-Chat-Completions conversion only extracted normal answer text, tools, and images, so type: "reasoning" output items and reasoning summary stream events were dropped from the Chat Completions payload.

That made reasoning-only upstream results look like empty assistant responses to clients that rely on Chat Completions semantics.

Changes

• Default adapted Chat Completions requests to reasoning: { effort: "medium", summary: "auto" } when the caller did not provide reasoning settings.
• Preserve explicit reasoning_effort while adding summary: "auto".
• Preserve explicit reasoning objects while adding a missing summary: "auto".
• Add shared extraction for Responses reasoning summary text from:
  • type: "reasoning" output items
  • summary[].text
  • reasoning summary delta/done stream events
• Map reasoning summaries to Chat Completions extension fields:
  • streaming: choices[].delta.reasoning and choices[].delta.reasoning_content
  • non-streaming: choices[].message.reasoning and choices[].message.reasoning_content
• Keep reasoning out of normal content, so user-visible answer text is not polluted.
• Preserve normal output text, images, and tool call behavior.

Validation

• cargo test -p codexmanager-service gateway::http_bridge::tests
  • 69 passed
• cargo test -p codexmanager-service gateway::local_validation::request::tests
  • 36 passed
• cargo test -p codexmanager-service
  • 950 passed
• cargo build -p codexmanager-service
  • passed

The branch is based directly on upstream/main and contains only this reasoning compatibility fix.