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

backend/internal/service/openai_model_mapping_test.go

@@ -69,14 +69,14 @@ func TestResolveOpenAIForwardModel(t *testing.T) {
 	}
 }
 
-func TestResolveOpenAIForwardModel_PreventsClaudeModelFromFallingBackToGpt51(t *testing.T) {
+func TestResolveOpenAIForwardModel_PreventsClaudeModelFromFallingBackToGpt54(t *testing.T) {
 	account := &Account{
 		Credentials: map[string]any{},
 	}
 
 	withoutDefault := normalizeCodexModel(resolveOpenAIForwardModel(account, "claude-opus-4-6", ""))
-	if withoutDefault != "gpt-5.1" {
-		t.Fatalf("normalizeCodexModel(...) = %q, want %q", withoutDefault, "gpt-5.1")
+	if withoutDefault != "gpt-5.4" {
+		t.Fatalf("normalizeCodexModel(...) = %q, want %q", withoutDefault, "gpt-5.4")
 	}
 
 	withDefault := normalizeCodexModel(resolveOpenAIForwardModel(account, "claude-opus-4-6", "gpt-5.4"))
@@ -87,9 +87,9 @@ func TestResolveOpenAIForwardModel_PreventsClaudeModelFromFallingBackToGpt51(t *
 
 func TestNormalizeCodexModel(t *testing.T) {
 	cases := map[string]string{
-		"gpt-5.3-codex-spark":       "gpt-5.3-codex",
-		"gpt-5.3-codex-spark-high":  "gpt-5.3-codex",
-		"gpt-5.3-codex-spark-xhigh": "gpt-5.3-codex",
+		"gpt-5.3-codex-spark":       "gpt-5.3-codex-spark",
+		"gpt-5.3-codex-spark-high":  "gpt-5.3-codex-spark",
+		"gpt-5.3-codex-spark-xhigh": "gpt-5.3-codex-spark",
 		"gpt-5.3":                   "gpt-5.3-codex",
 	}
 
@@ -111,7 +111,7 @@ func TestNormalizeOpenAIModelForUpstream(t *testing.T) {
 			name:    "oauth keeps codex normalization behavior",
 			account: &Account{Type: AccountTypeOAuth},
 			model:   "gemini-3-flash-preview",
-			want:    "gpt-5.1",
+			want:    "gpt-5.4",
 		},
 		{
 			name:    "apikey preserves custom compatible model",