修复Chat生成预算字段兼容

新增Chat生成预算字段能力并兼容新旧网关
VectorEngine编辑器客户端改用max_completion_tokens
补充BGM请求体回归与协议文档约束

scripts/test-ve-llm.mjs

@@ -87,7 +87,7 @@ const tests = [
     body: {
       model: 'gpt-5.4-mini',
       messages: [{ role: 'user', content: '回复 ok，不要解释' }],
-      max_tokens: 10,
+      max_completion_tokens: 10,
     },
   },
 
@@ -112,7 +112,7 @@ const tests = [
     body: {
       model: 'gpt-5.4-mini',
       messages: [{ role: 'user', content: '回复 ok' }],
-      max_tokens: 10,
+      max_completion_tokens: 10,
     },
   },
 
@@ -127,7 +127,7 @@ const tests = [
         { role: 'system', content: '你是抓大鹅游戏编辑，只返回 JSON。' },
         { role: 'user', content: '题材：水果。请生成 JSON：{"gameName":"水果切切乐","items":[{"name":"苹果","itemSize":"中"},{"name":"西瓜","itemSize":"大"}]}' },
       ],
-      max_tokens: 200,
+      max_completion_tokens: 200,
     },
   },
 ];