fix(review): gate capability proxy before provider send

[seonghobae]

Scope

Issue #1106 follow-up stacked on Draft #1235. An admitted free chat and rerank candidate could be reached through proxy_capability with an explicit model even while the review allocation contract was unavailable. The shared capability entry now checks that contract before candidate selection/provider send. The HTTP capability handler preserves the typed upstream error instead of converting it to capability_unavailable.

Evidence

• Independent review reproduced one mocked provider send at fix(review): fail closed without calibrated allocation evidence #1235 119367ea using synthetic rerank input.
• New direct and /v1/rerank regressions assert explicit model → nonretryable 400 review_model_not_allowed, orchestrator/free → nonretryable 503 allocation_evidence_unavailable, with zero provider sends.
• python -m pytest -q tests/test_review_gateway_admission_contract_1106.py tests/test_multimodal_model_group_http.py tests/test_provider_error_taxonomy.py: 87 passed (local; one pre-existing pytest config warning).
• docs/doctoring/review_gateway_supported_path_receipt.md records the separate supported runner path and why hosted before/after capacity numbers are unavailable. The earlier same-host synthetic receipt is boundary evidence only.

Draft/HOLD: no merge or deployment. Required checks, independent review, exact hosted build and comparable live request-path evidence remain open.

[coderabbitai]

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