feat: add OpenAI Responses API format conversion (api_format = "openai_responses")

Support Anthropic ↔ OpenAI Responses API format conversion alongside existing
Chat Completions conversion. The Responses API uses a flat input/output structure
with lifted function_call/function_call_output items and named SSE lifecycle events.

src/i18n/locales/en.json

@@ -697,6 +697,8 @@
     "apiFormatHint": "Select the input format for the provider's API",
     "apiFormatAnthropic": "Anthropic Messages (Native)",
     "apiFormatOpenAIChat": "OpenAI Chat Completions (Requires proxy)",
+    "apiFormatOpenAIResponses": "OpenAI Responses API (Requires proxy)",
+    "apiHintResponses": "💡 Fill in OpenAI Responses API compatible service endpoint, avoid trailing slash",
     "anthropicDefaultHaikuModel": "Default Haiku Model",
     "anthropicDefaultSonnetModel": "Default Sonnet Model",
     "anthropicDefaultOpusModel": "Default Opus Model",

src/i18n/locales/ja.json

@@ -697,6 +697,8 @@
     "apiFormatHint": "プロバイダー API の入力フォーマットを選択",
     "apiFormatAnthropic": "Anthropic Messages（ネイティブ）",
     "apiFormatOpenAIChat": "OpenAI Chat Completions（プロキシが必要）",
+    "apiFormatOpenAIResponses": "OpenAI Responses API（プロキシが必要）",
+    "apiHintResponses": "💡 OpenAI Responses API 互換サービスのエンドポイントを入力してください。末尾にスラッシュを付けないでください",
     "anthropicDefaultHaikuModel": "既定 Haiku モデル",
     "anthropicDefaultSonnetModel": "既定 Sonnet モデル",
     "anthropicDefaultOpusModel": "既定 Opus モデル",

src/i18n/locales/zh.json

@@ -697,6 +697,8 @@
     "apiFormatHint": "选择供应商 API 的输入格式",
     "apiFormatAnthropic": "Anthropic Messages (原生)",
     "apiFormatOpenAIChat": "OpenAI Chat Completions (需开启代理)",
+    "apiFormatOpenAIResponses": "OpenAI Responses API (需开启代理)",
+    "apiHintResponses": "💡 填写兼容 OpenAI Responses API 的服务端点地址，不要以斜杠结尾",
     "anthropicDefaultHaikuModel": "Haiku 默认模型",
     "anthropicDefaultSonnetModel": "Sonnet 默认模型",
     "anthropicDefaultOpusModel": "Opus 默认模型",