test: fix gh.spec.ts 404 error test by adding proper JSON response body

The test was returning an empty response body (null) which caused Octokit
to not properly parse the error status. Updated to return a proper JSON
error response with message and documentation_url, consistent with other
error handling tests in the file.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: snomiao

src/gh.spec.ts

@@ -347,10 +347,19 @@ describe("GitHub API Client (gh)", () => {
       // Mock a 404 response for lightweight tags
       server.use(
         http.get("https://api.github.com/repos/:owner/:repo/git/tags/:tag_sha", () => {
-          return new HttpResponse(null, {
-            status: 404,
-            statusText: "Not Found",
-          });
+          return new HttpResponse(
+            JSON.stringify({
+              message: "Not Found",
+              documentation_url: "https://docs.github.com/rest/reference/git#get-a-tag",
+            }),
+            {
+              status: 404,
+              statusText: "Not Found",
+              headers: {
+                "Content-Type": "application/json",
+              },
+            },
+          );
         }),
       );