refactor: 引入 Checker 统一接口与 Runner 抽象机制

定义 Checker 接口（resolve/execute/serialize）和 CheckerRegistry
注册中心，消除 engine/config-loader/store 中硬编码类型分支。
按 checker 类型分子包（runner/http/、runner/command/），提取
共享 expect 到 runner/shared/。超时控制通过引擎注入 AbortSignal。
CheckFailure.phase 从联合类型改为 string。配置校验下沉到各
Checker.resolve() 内部。

新增 checker-runner-abstraction spec，更新 DEVELOPMENT.md。

tests/server/checker/runner/shared/failure.test.ts

@@ -0,0 +1,79 @@
+import { describe, expect, test } from "bun:test";
+import { truncateActual, mismatchFailure, errorFailure } from "../../../../../src/server/checker/runner/shared/failure";
+
+describe("truncateActual", () => {
+  test("短字符串不截断", () => {
+    expect(truncateActual("hello")).toBe("hello");
+  });
+
+  test("恰好等于限制长度不截断", () => {
+    const str = "a".repeat(200);
+    expect(truncateActual(str)).toBe(str);
+  });
+
+  test("超过限制长度截断并加省略号", () => {
+    const str = "a".repeat(300);
+    const result = truncateActual(str) as string;
+    expect(result.length).toBe(203);
+    expect(result.endsWith("...")).toBe(true);
+    expect(result.startsWith("a".repeat(200))).toBe(true);
+  });
+
+  test("自定义最大长度", () => {
+    const str = "abcdefghij";
+    const result = truncateActual(str, 5) as string;
+    expect(result).toBe("abcde...");
+  });
+
+  test("null 不截断", () => {
+    expect(truncateActual(null)).toBe(null);
+  });
+
+  test("undefined 不截断", () => {
+    expect(truncateActual(undefined)).toBe(undefined);
+  });
+
+  test("数字转换为字符串后判断", () => {
+    expect(truncateActual(42)).toBe(42);
+    expect(truncateActual(123456789, 3) as string).toBe("123...");
+  });
+});
+
+describe("mismatchFailure", () => {
+  test("返回正确的 mismatch 结构", () => {
+    const f = mismatchFailure("status", "status", [200], 500, "status mismatch");
+    expect(f).toEqual({
+      kind: "mismatch",
+      phase: "status",
+      path: "status",
+      expected: [200],
+      actual: 500,
+      message: "status mismatch",
+    });
+  });
+
+  test("自动截断过长的 actual", () => {
+    const longStr = "x".repeat(300);
+    const f = mismatchFailure("body", "body[0]", "short", longStr, "too long");
+    expect((f.actual as string).endsWith("...")).toBe(true);
+    expect((f.actual as string).length).toBe(203);
+  });
+});
+
+describe("errorFailure", () => {
+  test("返回正确的 error 结构", () => {
+    const f = errorFailure("body", "body[0].json($.x)", "body is not valid JSON");
+    expect(f).toEqual({
+      kind: "error",
+      phase: "body",
+      path: "body[0].json($.x)",
+      message: "body is not valid JSON",
+    });
+  });
+
+  test("不含 expected 和 actual 字段", () => {
+    const f = errorFailure("headers", "headers.x", "header missing");
+    expect(f).not.toHaveProperty("expected");
+    expect(f).not.toHaveProperty("actual");
+  });
+});