feat: map anton's new StreamReasoningDelta to thought.progress subtype:reasoning (ENG-1109)

[torrmal]

Summary

Companion to mindsdb/anton#280, which adds a new StreamReasoningDelta event to Anton's provider layer and starts emitting it — for Claude's extended-thinking content blocks and OpenAI's reasoning-summary/reasoning_content — instead of either silently dropping it or letting narration text leak into the final answer.

This PR is the small piece that lets it actually reach the client: maps StreamReasoningDelta → response.in_progress with thought_role: thought.progress, content, subtype: reasoning in anton_harness/stream_formatter.py. That's the exact shape hermes_harness/stream_formatter.py already emits, and the exact shape the frontend's responseStreamAdapter.js already reads (built for cowork's ENG-1108, the ephemeral "current thought" UI) — so no frontend change is needed at all.

Linear: https://linear.app/mindsdb/issue/ENG-1109/anton-capture-claudeopenai-reasoning-as-a-real-subtypereasoning

Test plan

• Added tests/test_anton_stream_formatter.py — asserts a StreamReasoningDelta maps to the correct thought_role/subtype/content shape, and (critically) never leaks into response.output_text.delta alongside real answer text
• Ran against anton's local branch (feat/eng-1109-reasoning-subtype-channel, i.e. anton#280) via PYTHONPATH override to exercise the real StreamReasoningDelta type end-to-end — passes
• Full repo test suite not run in this session — it appeared to hang/hit port contention with a live local dev instance running for manual QA; recommend relying on CI here

🤖 Generated with Claude Code

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