fix: add per-model structured output opt-out

[Ingwannu]

Summary

• Preserve response_format translation as the default for routed openai-chat models.
• Add an explicit provider/model compatibility escape hatch, noStructuredOutputModels, so only exact listed model ids omit the field.
• Validate, normalize, persist, and expose the setting through the management configuration surfaces.
• Cover both Responses and Chat Completions ingress and document the compatibility boundary and decision.

Closes #1415

Verification

• nice -n 10 taskset -c 0-1 bun test tests/openai-chat-hardening.test.ts tests/chat-completions-endpoint.test.ts tests/management-provider-validation.test.ts — 138 pass, 0 fail.
• nice -n 10 taskset -c 0-1 bun run typecheck — passed.
• nice -n 10 taskset -c 0-1 bun run privacy:scan — passed.
• cd docs-site && bun install --frozen-lockfile && bun run build — passed, 221 pages built.
• nice -n 10 taskset -c 0-1 bun run test — 10,680 pass, 10 skip, 1 unrelated baseline failure in Codex autostart shim > Unix shim exports persisted service API token before running Codex.
• The same shim failure reproduces unchanged on exact base 9c051342d7ff7ad81b71911e359ad5935eaaf235 (34 pass, 1 fail in tests/codex-shim.test.ts); this patch does not modify that subsystem.
• Exact tested head: 9c88233a37a99e0eb93dc1555dc2d522761f3184.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[Wibias]

Request changes based on a full review of the current head.

Merge blockers / required fixes:

1. noStructuredOutputModels is documented and implemented as an exact per-model escape hatch, but the adapter uses modelInList(). That helper intentionally family-matches colon-tagged variants (foo also matches foo:variant). This can disable response_format for sibling/variant models that were not explicitly listed, which breaks the mixed-capability-gateway contract this PR is introducing. Use exact equality for this field (for example list.includes(parsed.modelId)) or a dedicated exact-match helper, and add a negative regression proving foo does not match foo:structured.

2. Persisted config.json and Management API PATCH normalize this field differently. PATCH trims and deduplicates entries, but schema/config loading only verifies that entry.trim() is nonblank and preserves the original string. A manually persisted value such as " deepseek-v4-flash " is accepted but will not match the runtime model id. Normalize trim/dedupe at the config boundary so every management surface has identical semantics.

I did not find a security vulnerability in this PR, but the first issue is a direct contract violation and should be fixed before merge. After fixes, refresh onto current dev and rerun exact-head CI.