feat(openai): 完善 Chat 与 Responses 参数转发

原生 Chat/Responses 改为透明转发，保留标准工具结构并保护调用方显式参数。补齐 Responses 到 Chat 的兼容转换、协议路由边界、完整响应和流式事件，并同步更新 Swagger、回归测试与真实验收脚本。

验证：
- cd apps/api && env -u AI_GATEWAY_TEST_DATABASE_URL go test ./... -count=1
- pnpm openapi
- pnpm lint
- pnpm test
- pnpm build
- gofmt -l 无输出
- git diff --check 通过

风险：
- Chat 回退无法等价表达的 Responses 原生能力现在会返回 unsupported_response_parameter
- 真实供应商 E2E 因本地没有已启用的平台模型候选而未完成

apps/api/internal/runner/responses.go

@@ -83,7 +83,8 @@ func (s *Service) prepareResponseExecution(ctx context.Context, task store.Gatew
 	return result, nil
 }
 
-func prepareResponseCandidates(candidates []store.RuntimeModelCandidate, execution responseExecutionContext) ([]store.RuntimeModelCandidate, error) {
+func prepareResponseCandidates(candidates []store.RuntimeModelCandidate, execution responseExecutionContext, body map[string]any) ([]store.RuntimeModelCandidate, error) {
+	chatCompatibilityErr := clients.ValidateResponsesChatFallback(body)
 	if execution.PreviousChain != nil {
 		for _, candidate := range candidates {
 			if candidate.PlatformModelID != execution.PreviousChain.PlatformModelID {
@@ -92,6 +93,9 @@ func prepareResponseCandidates(candidates []store.RuntimeModelCandidate, executi
 			if !candidateSupportsProtocol(candidate, execution.PreviousChain.UpstreamProtocol) {
 				break
 			}
+			if execution.PreviousChain.UpstreamProtocol == clients.ProtocolOpenAIChatCompletions && chatCompatibilityErr != nil {
+				return nil, chatCompatibilityErr
+			}
 			candidate.ResponseProtocol = execution.PreviousChain.UpstreamProtocol
 			return []store.RuntimeModelCandidate{candidate}, nil
 		}
@@ -106,6 +110,7 @@ func prepareResponseCandidates(candidates []store.RuntimeModelCandidate, executi
 	items := make([]indexedCandidate, 0, len(candidates))
 	hasAnthropicOnly := false
 	hasDeclaredUnsupported := false
+	hasChatCandidate := false
 	for index, candidate := range candidates {
 		protocols := candidateSupportedProtocols(candidate)
 		group := 1
@@ -119,11 +124,19 @@ func prepareResponseCandidates(candidates []store.RuntimeModelCandidate, executi
 				hasAnthropicOnly = true
 			}
 			continue
+		} else {
+			hasChatCandidate = true
+			if chatCompatibilityErr != nil {
+				continue
+			}
 		}
 		candidate.ResponseProtocol = protocol
 		items = append(items, indexedCandidate{candidate: candidate, index: index, group: group})
 	}
 	if len(items) == 0 {
+		if hasChatCandidate && chatCompatibilityErr != nil {
+			return nil, chatCompatibilityErr
+		}
 		if hasAnthropicOnly {
 			return nil, &clients.ClientError{Code: "unsupported_model_protocol", Message: "the selected model only supports Anthropic Messages; the public Anthropic adapter is not available", StatusCode: http.StatusBadRequest}
 		}