fix(openai): also match bare /responses route in handleStreamingAwareError

The first revision compared GetInboundEndpoint(c) against EndpointResponses
("/v1/responses"). NormalizeInboundEndpoint only recognizes paths that
contain the literal "/v1/responses" substring, but the project actually
registers six /responses routes — three of which (top-level
r.POST("/responses", ...) and codexDirect's "/backend-api/codex/responses")
have FullPath values without the "/v1" prefix and therefore fall through
to the default branch.

Codex CLI users targeting the bare /responses route at the production
deployment (observed 2026-05-24 ~11:05 UTC, user 16) never reached the
new writeResponsesFailedSSE path: the endpoint check was false, the
legacy `event: error` frame fired, and the strict SDK kept reporting
"stream closed before response.completed".

Replace the strict equality check with inboundIsResponses(c), which
uses suffix detection on FullPath (falling back to URL.Path when
FullPath is empty in test fixtures) and covers all six route variants:

  /v1/responses[/...]
  /responses[/...]
  /backend-api/codex/responses[/...]

Add test table covering all routes plus negative cases.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/handler/openai_gateway_handler.go

@@ -1695,7 +1695,7 @@ func (h *OpenAIGatewayHandler) handleStreamingAwareError(c *gin.Context, status
 		// response.completed/failed/incomplete/cancelled 集合。
 		// 通用 `event: error` 帧不被识别为终止事件，会导致
 		// "stream closed before response.completed"。
-		if GetInboundEndpoint(c) == EndpointResponses {
+		if inboundIsResponses(c) {
 			if writeResponsesFailedSSE(c, errType, message) {
 				return
 			}