fix(openai-gateway): address PR review — probe URL /v1 prefix, Create trigger, tests

Fix four issues flagged by copilot-pull-request-reviewer on PR #2143:

1. Probe URL missing /v1 prefix (openai_apikey_responses_probe.go)
   Replaced bare TrimSuffix + "/responses" with buildOpenAIResponsesURL(),
   which handles bare domain → /v1/responses correctly. Affected:
   - ProbeOpenAIAPIKeyResponsesSupport (probe URL)
   - TestAccount endpoint (apiURL for APIKey accounts)

2. Create endpoint not triggering probe (account_handler.go)
   Capture created account from idempotent closure and call
   scheduleOpenAIResponsesProbe after success, same pattern as
   BatchCreate and Update.

3. Tests (openai_gateway_chat_completions_raw_test.go)
   Added TestBuildOpenAIChatCompletionsURL (7 cases covering
   bare domain, /v1 suffix, trailing slash, third-party domains,
   whitespace) and TestBuildOpenAIResponsesURL_ProbeURL (6 cases
   locking the probe URL construction for bare-domain inputs).

All unit tests pass; go build ./cmd/server/ clean.

backend/internal/service/openai_apikey_responses_probe.go

@@ -85,7 +85,7 @@ func (s *AccountTestService) ProbeOpenAIAPIKeyResponsesSupport(ctx context.Conte
 		return
 	}
 
-	probeURL := strings.TrimSuffix(normalizedBaseURL, "/") + "/responses"
+	probeURL := buildOpenAIResponsesURL(normalizedBaseURL)
 
 	probeCtx, cancel := context.WithTimeout(ctx, openaiResponsesProbeTimeout)
 	defer cancel()