feat(gateway): add OpenResponses /v1/responses endpoint

Add a new `/v1/responses` endpoint implementing the OpenResponses API
standard for agentic workflows. This provides:

- Item-based input (messages, function_call_output, reasoning)
- Semantic streaming events (response.created, response.output_text.delta,
  response.completed, etc.)
- Full SSE event support with both event: and data: lines
- Configuration via gateway.http.endpoints.responses.enabled

The endpoint is disabled by default and can be enabled independently
from the existing Chat Completions endpoint.

Phase 1 implementation supports:
- String or ItemParam[] input
- system/developer/user/assistant message roles
- function_call_output items
- instructions parameter
- Agent routing via headers or model parameter
- Session key management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gateway/server-runtime-state.ts

@@ -27,6 +27,7 @@ export async function createGatewayRuntimeState(params: {
   controlUiEnabled: boolean;
   controlUiBasePath: string;
   openAiChatCompletionsEnabled: boolean;
+  openResponsesEnabled: boolean;
   resolvedAuth: ResolvedGatewayAuth;
   gatewayTls?: GatewayTlsRuntime;
   hooksConfig: () => HooksConfigResolved | null;
@@ -103,6 +104,7 @@ export async function createGatewayRuntimeState(params: {
     controlUiEnabled: params.controlUiEnabled,
     controlUiBasePath: params.controlUiBasePath,
     openAiChatCompletionsEnabled: params.openAiChatCompletionsEnabled,
+    openResponsesEnabled: params.openResponsesEnabled,
     handleHooksRequest,
     handlePluginRequest,
     resolvedAuth: params.resolvedAuth,