feat: add Responses API compatibility

apps/api/internal/store/candidates.go

@@ -31,7 +31,18 @@ func (s *Store) ListModelCandidates(ctx context.Context, model string, modelType
        COALESCE(p.dynamic_priority, p.priority) AS effective_priority,
        m.id::text, COALESCE(m.base_model_id::text, ''), COALESCE(b.canonical_model_key, ''),
        COALESCE(NULLIF(m.provider_model_name, ''), m.model_name), m.model_name, COALESCE(m.model_alias, ''),
-       $2::text AS requested_model_type, m.display_name, m.capabilities, m.capability_override,
+       $2::text AS requested_model_type, m.display_name,
+       CASE
+         WHEN b.capabilities #> '{text_generate,supportedApiProtocols}' IS NOT NULL
+         THEN jsonb_set(
+           COALESCE(m.capabilities, '{}'::jsonb),
+           '{text_generate,supportedApiProtocols}',
+           b.capabilities #> '{text_generate,supportedApiProtocols}',
+           true
+         )
+         ELSE m.capabilities
+       END AS effective_capabilities,
+       m.capability_override,
        COALESCE(b.base_billing_config, '{}'::jsonb), m.billing_config, m.billing_config_override,
        m.pricing_mode, COALESCE(m.discount_factor, 0)::float8, COALESCE(m.pricing_rule_set_id::text, ''),
        COALESCE(b.pricing_rule_set_id::text, ''),