fix(api): expose per-member supportsVision on /v1/combos model steps (#14587)

[diegosouzapw]

Closes #14587

Root cause

The core of #14587 (vision override honored, MiMo id fragments) already landed via #14595 / #15086; getResolvedModelCapabilities now agrees with routing on the modelCompatOverrides.supportsVision row (covered by tests/unit/model-compat-vision-canonical-resolution-14587.test.ts). What remained was diagnosability: /v1/combos only exposed the aggregate capabilities.multimodal, so an operator could not tell which member blocks it.

Fix

• PublicComboStep gains supportsVision?: boolean | null (true/false/null = no source knows), set on direct model steps when capabilities are included, using the same resolver (and bulk snapshot) as the aggregate. No behavior change to multimodal.
• docs/openapi.yaml documents the field on GET /api/v1/combos.

Tests

tests/unit/14587-combo-member-supports-vision.test.ts (new): per-member verdicts, invariant multimodal === every(member.supportsVision === true), field omitted without capabilities.

• RED before fix: 2 fail (field absent) / 1 pass.
• GREEN after: 3/3. Also green: v1-combos-projection (14), model-compat-vision-canonical-resolution-14587 (6).

Gates

eslint (with suppressions) exit 0, prettier, typecheck:core exit 0, check-complexity OK (3003 < 3218), check-cognitive-complexity OK (1374 < 1437), check-file-size no violations, husky pre-commit gates passed.

⚠️ base-red inherited: #15306 — unit/integration/package-artifact gates timing out, vitest open-sse/mcp-server/tests/audit.test.ts, check:ai-attribution range error

[github-actions]

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