feat(codex): switch Volcengine Agentplan preset to native Responses

Official Codex docs (volcengine.com/docs/82379/2556056, updated
2026-07) confirm the Coding Plan endpoint /api/coding/v3 supports the
Responses API, so the preset no longer needs local route conversion.
BytePlus stays on Chat routing until the international-site docs are
verified. Also document the billing pitfall: the pay-as-you-go /api/v3
endpoint must never appear in plan-subscription endpoint candidates.

tests/config/codexChatProviderPresets.test.ts

@@ -10,13 +10,8 @@ const expectedChatPresets = new Map<
   string,
   { baseUrl: string; contextWindows: Record<string, number> }
 >([
-  [
-    "火山Agentplan",
-    {
-      baseUrl: "https://ark.cn-beijing.volces.com/api/coding/v3",
-      contextWindows: { "ark-code-latest": 256000 },
-    },
-  ],
+  // 火山Agentplan（国内站 coding/v3）已切原生 Responses，见下方 native 清单；
+  // BytePlus 国际站未核实，保持 Chat 路由
   [
     "BytePlus",
     {
@@ -156,6 +151,8 @@ describe("Codex Chat provider presets", () => {
       string,
       { contextWindows: Record<string, number> }
     >([
+      // 官方 Codex 文档确认 Coding Plan /api/coding/v3 支持 Responses API
+      ["火山Agentplan", { contextWindows: { "ark-code-latest": 256000 } }],
       [
         "DouBaoSeed",
         { contextWindows: { "doubao-seed-2-1-pro-260628": 262144 } },