fix(openai): 移除已下线 Codex 模型并修复归一化兜底副作用

- backend: 删除 gpt-5 / 5.1 / 5.1-codex / 5.1-codex-max / 5.1-codex-mini / 5.2-codex / 5.4-nano 的内置映射与 DefaultModels 条目
- backend: normalizeCodexModel 默认兜底由 gpt-5.1 改为 gpt-5.4，gpt-5.3-codex-spark 独立保留映射
- backend: 修复 isOpenAIGPT54Model 与 shouldAutoInjectPromptCacheKeyForCompat 对 claude / gpt-4o 的误判（之前依赖 gpt-5.1 作为非 GPT 族的隐式 sentinel，改后需要显式前缀守卫）
- backend: 清理 billing_service 中已不可达的 fallback 价格与 switch 分支
- frontend: 从白名单、OpenCode 配置、预设映射中移除已下线模型
- 同步更新所有相关单测

Refs: #1758, parallels upstream #1759 but adds downstream guard fixes

frontend/src/components/keys/UseKeyModal.vue

@@ -617,66 +617,6 @@ function generateOpenCodeConfig(platform: string, baseUrl: string, apiKey: strin
     }
   }
   const openaiModels = {
-    'gpt-5-codex': {
-      name: 'GPT-5 Codex',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {}
-      }
-    },
-    'gpt-5.1-codex': {
-      name: 'GPT-5.1 Codex',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {}
-      }
-    },
-    'gpt-5.1-codex-max': {
-      name: 'GPT-5.1 Codex Max',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {}
-      }
-    },
-    'gpt-5.1-codex-mini': {
-      name: 'GPT-5.1 Codex Mini',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {}
-      }
-    },
     'gpt-5.2': {
       name: 'GPT-5.2',
       limit: {
@@ -725,22 +665,6 @@ function generateOpenCodeConfig(platform: string, baseUrl: string, apiKey: strin
         xhigh: {}
       }
     },
-    'gpt-5.4-nano': {
-      name: 'GPT-5.4 Nano',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {},
-        xhigh: {}
-      }
-    },
     'gpt-5.3-codex-spark': {
       name: 'GPT-5.3 Codex Spark',
       limit: {
@@ -773,22 +697,6 @@ function generateOpenCodeConfig(platform: string, baseUrl: string, apiKey: strin
         xhigh: {}
       }
     },
-    'gpt-5.2-codex': {
-      name: 'GPT-5.2 Codex',
-      limit: {
-        context: 400000,
-        output: 128000
-      },
-      options: {
-        store: false
-      },
-      variants: {
-        low: {},
-        medium: {},
-        high: {},
-        xhigh: {}
-      }
-    },
     'codex-mini-latest': {
       name: 'Codex Mini',
       limit: {