fix: cap Responses image candidates

[Minorli]

Summary

• force and serialize Responses image tool calls with an explicit requested-count instruction
• remove unsupported image_generation n and its non-idempotent retry path
• cap successfully renderable candidates before extra files are persisted
• release version 0.1.58

Production evidence

• reproduced sample_count=1 returning 6 image tool calls in 395.3 seconds
• compatible gateway rejects official max_tool_calls with 502
• verified tool_choice + parallel_tool_calls=false + exact-count instruction against gpt-5.6-sol/xhigh: 200, one image call, 28.4 seconds

Verification

• ./scripts/check.sh (386 passed)
• coverage: 87%
• node --check static/app.js
• bash -n scripts/*.sh
• docker compose config --quiet
• uv lock --check
• local health/readiness/static-version smoke test