feat(openai): 增加 gpt-5.4-mini/nano 模型支持与定价配置

- 接入 gpt-5.4-mini/nano 模型识别与规范化，补充默认模型列表
- 增加 gpt-5.4-mini/nano 输入/缓存命中/输出价格与计费兜底逻辑
- 同步前端模型白名单与 OpenCode 配置
- 补充 service tier(priority/flex) 计费回归测试

backend/resources/model-pricing/model_prices_and_context_window.json

@@ -5173,6 +5173,71 @@
     "supports_tool_choice": true,
     "supports_vision": true
   },
+  "gpt-5.4-mini": {
+    "cache_read_input_token_cost": 7.5e-08,
+    "input_cost_per_token": 7.5e-07,
+    "litellm_provider": "openai",
+    "max_input_tokens": 400000,
+    "max_output_tokens": 128000,
+    "max_tokens": 128000,
+    "mode": "chat",
+    "output_cost_per_token": 4.5e-06,
+    "supported_endpoints": [
+      "/v1/chat/completions",
+      "/v1/batch",
+      "/v1/responses"
+    ],
+    "supported_modalities": [
+      "text",
+      "image"
+    ],
+    "supported_output_modalities": [
+      "text"
+    ],
+    "supports_function_calling": true,
+    "supports_native_streaming": true,
+    "supports_parallel_function_calling": true,
+    "supports_pdf_input": true,
+    "supports_prompt_caching": true,
+    "supports_reasoning": true,
+    "supports_response_schema": true,
+    "supports_service_tier": true,
+    "supports_system_messages": true,
+    "supports_tool_choice": true,
+    "supports_vision": true
+  },
+  "gpt-5.4-nano": {
+    "cache_read_input_token_cost": 2e-08,
+    "input_cost_per_token": 2e-07,
+    "litellm_provider": "openai",
+    "max_input_tokens": 400000,
+    "max_output_tokens": 128000,
+    "max_tokens": 128000,
+    "mode": "chat",
+    "output_cost_per_token": 1.25e-06,
+    "supported_endpoints": [
+      "/v1/chat/completions",
+      "/v1/batch",
+      "/v1/responses"
+    ],
+    "supported_modalities": [
+      "text",
+      "image"
+    ],
+    "supported_output_modalities": [
+      "text"
+    ],
+    "supports_function_calling": true,
+    "supports_native_streaming": true,
+    "supports_parallel_function_calling": true,
+    "supports_pdf_input": true,
+    "supports_prompt_caching": true,
+    "supports_reasoning": true,
+    "supports_response_schema": true,
+    "supports_system_messages": true,
+    "supports_tool_choice": true,
+    "supports_vision": true
+  },
   "gpt-5.3-codex": {
     "cache_read_input_token_cost": 1.75e-07,
     "cache_read_input_token_cost_priority": 3.5e-07,