fix(codex): accumulate output items from stream events

[grearsl]

Summary

• The Codex/OpenAI Responses streaming API sends output items via response.output_item.done events, but the final response.completed event arrives with an empty Output array
• This caused all Codex OAuth provider responses to return empty content (0 content chars) despite the model successfully generating a reply (completion tokens > 0)
• Fix accumulates output items as they stream in and merges them into the response when response.completed has none

Reproduction

1. Configure a model with auth_method: "oauth" (Codex provider)
2. Send any message
3. Response returns "The model returned an empty response" despite successful API call

Root cause

In codex_provider.go, the streaming loop only captured evt.Response from terminal events (response.completed), but the OpenAI Responses API does not populate Response.Output in that event — output items are delivered separately via response.output_item.done events.

Test plan

• Tested with gpt-5.4 via OAuth — responses now returned correctly
• Tested with gpt-5.3-codex via OAuth — same fix applies
• Verified tool calls and text content both parse correctly
• No regressions on non-Codex providers (change is scoped to codex_provider.go)

🤖 Generated with Claude Code

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