fix(codex): preserve streamed output text deltas

[miruchigawa]

📝 Description

Fixes OpenAI/Codex OAuth empty responses when the backend streams valid text through response.output_text.delta but sends a final response.completed event with response.output set to null.

The Codex provider now accumulates streamed output_text delta events while reading the response stream. If the final parsed response has no content, PicoClaw falls back to the accumulated streamed text instead of reporting a false empty model response.

This also adds a regression test for the issue #2953 stream shape: response.output_text.delta contains OK, followed by response.completed with null output.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #2953

📚 Technical Context (Skip for Docs)

• Reference URL: https://developers.openai.com/api/docs/guides/streaming-responses?api-mode=responses
• Reasoning: OpenAI Responses streaming is event-driven, and text can arrive incrementally in response.output_text.delta events. The final response.completed event should be treated as stream completion; for Codex OAuth it may not include populated response.output, so PicoClaw needs to preserve streamed deltas before parsing the final response.

� Test Environment

• Hardware: Linux development workspace / PC
• OS: Linux
• Model/Provider: OpenAI/Codex OAuth (gpt-5.3-codex issue path; local regression uses mocked Codex Responses stream)
• Channels: CLI agent

📸 Evidence (Optional)

Click to view Logs/Screenshots

go test ./pkg/providers/oauth -run TestCodexProvider_ChatRoundTrip_OutputTextDeltaFallback -count=1
# ok   github.com/sipeed/picoclaw/pkg/providers/oauth

go test ./pkg/providers/oauth -count=1
# ok   github.com/sipeed/picoclaw/pkg/providers/oauth

make build
# Build complete: build/picoclaw-linux-amd64
# Build complete: build/picoclaw

./build/picoclaw agent -m "hai" --model "gpt-5.4" --no-color
# ██████╗ ██╗ ██████╗ ██████╗  ██████╗██╗      # █████╗ ██╗    ██╗
# ██╔══██╗██║██╔════╝██╔═══██╗██╔════╝██# ║     ██╔══██╗██║    ██║
# ██████╔╝██║██║     ██║   ██║██║     ██║     # ███████║██║ █╗ ██║
# ██╔═══╝ ██║██║     ██║   ██║██║     ██║     # ██╔══██║██║███╗██║
# ██║     # ██║╚██████╗╚██████╔╝╚██████╗███████╗██# ║  ██║╚███╔███╔╝
# ╚═╝     ╚═╝ ╚═════╝ ╚═════╝  # ╚═════╝╚══════╝╚═╝  ╚═╝ ╚══╝╚══╝
#
#
# � hai 😄 what’s up?

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

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All committers have signed the CLA.

[afjcjsbx]

@miruchigawa Hi thanks for the PR, pls fix lint 🙏

[miruchigawa]

@afjcjsbx pls review it asap 🔥