Fix Codex OAuth empty responses for gpt-5.4

[Pepelospalotes2005]

Summary

Fix empty-response handling for Codex/OpenAI OAuth gpt-5.4 by improving fallback extraction when the final SDK response arrives completed but without hydrated output items.

Problem

With OpenAI OAuth connected and model gpt-5.4, PicoClaw could reach the Codex backend successfully, consume tokens, and still return an empty assistant response.

Observed behavior:

• OAuth credentials were saved correctly
• auth_method = oauth was applied correctly to openai/gpt-5.4
• the runtime entered the Codex/OpenAI OAuth path
• the backend completed the request
• but PicoClaw ended with content_len=0 and surfaced the fallback:
  • The model returned an empty response. This may indicate a provider error or token limit.

Root cause

Two related response-shape issues:

1. responses.Response could arrive completed but with no hydrated Output items
2. streaming text deltas could carry usable text even when the final Response object remained effectively empty

Fix

1. Responses parser fallback

In pkg/providers/openai_responses_common/responses_common.go:

• fallback to apiResp.OutputText() when the manual Output traversal produces empty content

2. Codex streaming fallback

In pkg/providers/codex_provider.go:

• accumulate stream events from:
  • response.output_text.delta
  • response.output_text.done
  • response.reasoning_text.delta
• use those accumulated buffers as fallback when the final parsed response still comes back empty

Validation

Validated on a live ARM64 Linux VPS running the forked binary behind the real launcher/dashboard flow:

• OAuth login works
• gpt-5.4 now returns actual text instead of the empty-response fallback

Notes

This PR intentionally excludes local ARM64 Matrix/libolm build workarounds used only for building on the test VPS.

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