refactor(web): hasApplicableFix 改类型谓词，消除 applyConflictFix 强转

hasApplicableFix 重写为对象入参类型谓词 patch is ApplicablePatch，谓词命中后 applyConflictFix 里 original/replacement 自动收窄为 string，删两处 as string；补收窄单测。

apps/web/lib/review/applyFix.test.ts

@@ -4,12 +4,26 @@ import { applyConflictFix, hasApplicableFix } from "./applyFix";
 
 describe("hasApplicableFix", () => {
   it("requires non-empty original and string replacement", () => {
-    expect(hasApplicableFix("迈巴赫", "奔驰")).toBe(true);
-    expect(hasApplicableFix("删我", "")).toBe(true); // 空串=删除，仍可应用
-    expect(hasApplicableFix(null, "奔驰")).toBe(false);
-    expect(hasApplicableFix("", "奔驰")).toBe(false);
-    expect(hasApplicableFix("迈巴赫", null)).toBe(false);
-    expect(hasApplicableFix(undefined, undefined)).toBe(false);
+    expect(hasApplicableFix({ original: "迈巴赫", replacement: "奔驰" })).toBe(true);
+    // 空串=删除，仍可应用
+    expect(hasApplicableFix({ original: "删我", replacement: "" })).toBe(true);
+    expect(hasApplicableFix({ original: null, replacement: "奔驰" })).toBe(false);
+    expect(hasApplicableFix({ original: "", replacement: "奔驰" })).toBe(false);
+    expect(hasApplicableFix({ original: "迈巴赫", replacement: null })).toBe(false);
+    expect(hasApplicableFix({ original: undefined, replacement: undefined })).toBe(
+      false,
+    );
+  });
+
+  it("narrows original/replacement to string when true (type predicate)", () => {
+    const patch: { original: string | null; replacement: string | null } = {
+      original: "迈巴赫",
+      replacement: "奔驰",
+    };
+    if (!hasApplicableFix(patch)) throw new Error("expected applicable");
+    // 编译期收窄：无需强转即可当 string 拼接（`as string` 已消除）。
+    const combined: string = patch.original + patch.replacement;
+    expect(combined).toBe("迈巴赫奔驰");
   });
 });