[openai-agents / OpenRouter] OpenRouterResponses injects additionalProperties:false into schemas for non-OpenAI providers

## Observed symptom [Bug] OpenRouterResponses injects additionalProperties: false into schemas for non-OpenAI providers (xAI/Grok rejects with 400) Copied from https://github.com/agno-agi/agno/issues/7455 When using `OpenRouterResponses` with `x-ai/grok-4.20`, all requests with `output_schema` (structured output) are rejected by the xAI provider with HTTP 400: ## Literal error text ``` Schema validation failed: [standard_violation] /properties/properties/additionalProperties: ``` ## Environment GitHub issue DeadantCo/agno #469; state=open ## Trigger / context Public GitHub report in DeadantCo/agno (FRESH_7D). ## Source evidence - https://github.com/DeadantCo/agno/issues/469 (2026-09-28T19:26:49Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #469 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:39:37.643698Z - scout_id: C2dc7df0ae0 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.