refactor: 移除 defaults.http.method 配置，简化默认值体系

- HTTP checker defaults schema 不再支持 method 字段
- resolve 逻辑从三级 fallback 简化为两级（target -> 内置默认）
- 配置文件中出现 defaults.http.method 将触发未知字段校验错误
- per-target http.method 覆盖功能保持不变
- 同步更新示例配置、README 文档和测试用例

src/server/checker/runner/http/execute.ts

@@ -105,9 +105,9 @@ export class HttpChecker implements CheckerDefinition<ResolvedHttpTarget> {
     const t = target as RawTargetConfig & { http: HttpTargetConfig; type: "http" };
     const httpDefaults = context.defaults["http"] as
       | undefined
-      | { headers?: Record<string, string>; maxBodyBytes?: string; method?: string };
+      | { headers?: Record<string, string>; maxBodyBytes?: string };
 
-    const method = t.http.method ?? httpDefaults?.method ?? "GET";
+    const method = t.http.method ?? "GET";
     const maxBodyBytes = parseSize(t.http.maxBodyBytes ?? httpDefaults?.maxBodyBytes ?? "100MB");
 
     return {