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

tests/server/checker/config-loader.test.ts

@@ -173,7 +173,6 @@ defaults:
   interval: "15s"
   timeout: "5s"
   http:
-    method: "POST"
     headers:
       Authorization: "Bearer token"
     maxBodyBytes: "50MB"
@@ -186,6 +185,7 @@ targets:
     interval: "1m"
     http:
       url: "http://example.com"
+      method: "POST"
       ignoreSSL: true
       maxRedirects: 5
     expect:
@@ -255,7 +255,6 @@ targets:
   interval: "30s"
   timeout: "10s"
   http:
-    method: "GET"
     maxBodyBytes: "10MB"
 targets:
   - name: "override-all"
@@ -1038,19 +1037,19 @@ targets:
     );
   });
 
-  test("defaults.http.method 小写输入失败", async () => {
+  test("defaults.http.method 触发未知字段错误", async () => {
     await expectConfigError(
-      "lowercase-default-method.yaml",
+      "unknown-default-method.yaml",
       `defaults:
   http:
-    method: post
+    method: POST
 targets:
   - name: "test"
     type: http
     http:
       url: "http://example.com"
 `,
-      "defaults.http.method 不在允许范围内",
+      "defaults.http.method 是未知字段",
     );
   });