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

frontend/src/composables/__tests__/useModelWhitelist.spec.ts

@@ -11,6 +11,8 @@ describe('useModelWhitelist', () => {
     const models = getModelsByPlatform('openai')
 
     expect(models).toContain('gpt-5.4')
+    expect(models).toContain('gpt-5.4-mini')
+    expect(models).toContain('gpt-5.4-nano')
     expect(models).toContain('gpt-5.4-2026-03-05')
   })
 
@@ -52,4 +54,13 @@ describe('useModelWhitelist', () => {
       'gpt-5.4-2026-03-05': 'gpt-5.4-2026-03-05'
     })
   })
+
+  it('whitelist keeps GPT-5.4 mini and nano exact mappings', () => {
+    const mapping = buildModelMappingObject('whitelist', ['gpt-5.4-mini', 'gpt-5.4-nano'], [])
+
+    expect(mapping).toEqual({
+      'gpt-5.4-mini': 'gpt-5.4-mini',
+      'gpt-5.4-nano': 'gpt-5.4-nano'
+    })
+  })
 })

frontend/src/composables/useModelWhitelist.ts

@@ -25,7 +25,7 @@ const openaiModels = [
   'gpt-5.2', 'gpt-5.2-2025-12-11', 'gpt-5.2-chat-latest',
   'gpt-5.2-codex', 'gpt-5.2-pro', 'gpt-5.2-pro-2025-12-11',
   // GPT-5.4 系列
-  'gpt-5.4', 'gpt-5.4-2026-03-05',
+  'gpt-5.4', 'gpt-5.4-mini', 'gpt-5.4-nano', 'gpt-5.4-2026-03-05',
   // GPT-5.3 系列
   'gpt-5.3-codex', 'gpt-5.3-codex-spark',
   'chatgpt-4o-latest',