feat: 扩展配置变量替换范围至 server/probes/targets，支持空默认值语法

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

@@ -436,6 +436,49 @@ targets:
     expect(target.http.maxRedirects).toBe(5);
   });
 
+  test("server 和 probes 在 schema 校验前完成变量替换", async () => {
+    const configPath = join(tempDir, "variables-server-probes.yaml");
+    await writeFile(
+      configPath,
+      `variables:
+  server_host: "0.0.0.0"
+  server_port: 3100
+  retention: "24h"
+  log_level: "debug"
+  rotation_max_files: 30
+  max_checks: 5
+server:
+  listen:
+    host: "\${server_host}"
+    port: "\${server_port}"
+  storage:
+    retention: "\${retention}"
+  logging:
+    level: "\${log_level}"
+    file:
+      rotation:
+        maxFiles: "\${rotation_max_files}"
+probes:
+  execution:
+    maxConcurrentChecks: "\${max_checks}"
+targets:
+  - id: "server-vars"
+    type: http
+    http:
+      url: "http://example.com"
+`,
+    );
+
+    const config = await loadConfig(configPath);
+    expect(config.host).toBe("0.0.0.0");
+    expect(config.port).toBe(3100);
+    expect(config.retentionMs).toBe(86400000);
+    expect(config.logging.consoleLevel).toBe("debug");
+    expect(config.logging.fileLevel).toBe("debug");
+    expect(config.logging.rotationMaxFiles).toBe(30);
+    expect(config.maxConcurrentChecks).toBe(5);
+  });
+
   test("变量替换后类型不匹配导致 schema 校验失败", async () => {
     const configPath = join(tempDir, "bad-var-type.yaml");
     await writeFile(
@@ -494,6 +537,38 @@ targets:
     await expectConfigLoadError(configPath, "未定义的变量");
   });
 
+  test("server 和 probes 中未定义变量阻止启动并输出字段路径", async () => {
+    await expectConfigError(
+      "unresolved-server-probes.yaml",
+      `server:
+  listen:
+    host: "\${MISSING_SERVER_HOST}"
+probes:
+  execution:
+    maxConcurrentChecks: "\${MISSING_MAX_CHECKS}"
+targets:
+  - id: "unresolved-root"
+    type: http
+    http:
+      url: "http://example.com"
+`,
+      "server.listen.host",
+    );
+    await expectConfigError(
+      "unresolved-probes.yaml",
+      `probes:
+  execution:
+    maxConcurrentChecks: "\${MISSING_MAX_CHECKS}"
+targets:
+  - id: "unresolved-probes"
+    type: http
+    http:
+      url: "http://example.com"
+`,
+      "probes.execution.maxConcurrentChecks",
+    );
+  });
+
   test("绝对 dataDir 保持不变", async () => {
     const dataDir = join(tempDir, "absolute-data");
     const configPath = join(tempDir, "absolute-data-dir.yaml");