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/httpapi/chat_completions_mode_test.go

@@ -294,7 +294,7 @@ func TestWriteCompatibleTaskResponseStreamsStructuredToolAndReasoningDeltas(t *t
 	if roleIndex < 0 || reasoningIndex < 0 || roleIndex > reasoningIndex {
 		t.Fatalf("assistant role should be emitted before structured deltas: %s", body)
 	}
-	for _, want := range []string{`"system_fingerprint":"fp-test"`, `"created":1710000000`, `"reasoning_content":"tagged"`, `"content":"answer"`, `"tool_calls":[{"function":{"arguments":"{\"city\":\"Boston\"}","name":"legacy_lookup"}`, `"tool_calls":[{"function":{"arguments":"{\"q\":"`, `"finish_reason":"tool_calls"`, `"choices":[],"created":1710000000`, `"usage":{"completion_tokens":5,"prompt_tokens":4,"total_tokens":9}`, "data: [DONE]"} {
+	for _, want := range []string{`"system_fingerprint":"fp-test"`, `"created":1710000000`, `"reasoning_content":"tagged"`, `"content":"answer"`, `"function_call":{"arguments":"{\"city\":\"Boston\"}","name":"legacy_lookup"}`, `"tool_calls":[{"function":{"arguments":"{\"q\":"`, `"finish_reason":"tool_calls"`, `"choices":[],"created":1710000000`, `"usage":{"completion_tokens":5,"prompt_tokens":4,"total_tokens":9}`, "data: [DONE]"} {
 		if !strings.Contains(body, want) {
 			t.Fatalf("SSE body missing %s: %s", want, body)
 		}