Fix upgrade silently replacing wrong binary (execPath bug) and swallowing errors

Root cause: in some compiled Bun versions, process.execPath returns the Bun
runtime path (~/.bun/bin/bun) or an internal /$bunfs/ path instead of the
deployed binary path. This causes the mv to either fail (text file busy) or
replace the wrong file, with the error going silently to stderr.

Fixes:
- github-code-search.ts: derive selfPath with a guard — if process.execPath
  starts with '/$bunfs/' (internal Bun path) fall back to resolve(argv[0]).
  Wrap performUpgrade in a try/catch that prints errors to STDOUT so they are
  always visible.
- upgrade.ts (downloadBinary): read response body explicitly with
  arrayBuffer() instead of passing the Response object to Bun.write (avoids
  edge-case body streaming issues). Validate the downloaded size is non-zero.
  Print a 'Replacing <dest>...' diagnostic so the target path is visible.
- upgrade.test.ts: add tests for downloadBinary paths (non-ok download,
  empty body, full success) to keep coverage above the 75% threshold.

Refs #45

Author: shouze

src/upgrade.test.ts

@@ -218,3 +218,76 @@ describe("performUpgrade", () => {
     );
   });
 });
+
+// ─── performUpgrade — download path (covers downloadBinary) ──────────────────
+
+describe("performUpgrade — download path", () => {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const origBunWrite = (Bun as any).write;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const origBunSpawnSync = (Bun as any).spawnSync;
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (Bun as any).write = origBunWrite;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (Bun as any).spawnSync = origBunSpawnSync;
+  });
+
+  /** Returns a release mock + matching asset name for the current platform. */
+  function mockReleaseAndDownload(downloadResponse: Response): void {
+    const platformMap: Record<string, string> = { darwin: "macos", win32: "windows" };
+    const p = platformMap[process.platform] ?? process.platform;
+    const suffix = p === "windows" ? ".exe" : "";
+    const assetName = `github-code-search-${p}-${process.arch}${suffix}`;
+    let callCount = 0;
+    globalThis.fetch = (async () => {
+      callCount++;
+      if (callCount === 1) {
+        return new Response(
+          JSON.stringify({ tag_name: "v9.9.9", assets: [makeAsset(assetName)] }),
+          {
+            status: 200,
+            headers: { "content-type": "application/json" },
+          },
+        );
+      }
+      return downloadResponse;
+    }) as typeof fetch;
+  }
+
+  it("throws when the binary download returns a non-OK status", async () => {
+    mockReleaseAndDownload(new Response("Bad Gateway", { status: 502 }));
+    await expect(performUpgrade("1.0.0", "/tmp/gcs-test-nonok")).rejects.toThrow(
+      "Download failed (502)",
+    );
+  });
+
+  it("throws when the downloaded binary is empty", async () => {
+    mockReleaseAndDownload(new Response(new ArrayBuffer(0), { status: 200 }));
+    await expect(performUpgrade("1.0.0", "/tmp/gcs-test-empty")).rejects.toThrow("empty file");
+  });
+
+  it("prints Upgrading and Successfully upgraded on a successful full upgrade", async () => {
+    mockReleaseAndDownload(new Response(new Uint8Array([1, 2, 3]).buffer, { status: 200 }));
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (Bun as any).write = async () => 3;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (Bun as any).spawnSync = () => ({ exitCode: 0, stderr: { toString: () => "" } });
+
+    const stdoutWrites: string[] = [];
+    const origWrite = process.stdout.write.bind(process.stdout);
+    process.stdout.write = ((s: string) => {
+      stdoutWrites.push(s);
+      return true;
+    }) as typeof process.stdout.write;
+
+    await performUpgrade("1.0.0", "/tmp/gcs-test-success");
+    process.stdout.write = origWrite;
+
+    expect(stdoutWrites.some((s) => s.includes("Upgrading"))).toBe(true);
+    expect(stdoutWrites.some((s) => s.includes("Replacing"))).toBe(true);
+    expect(stdoutWrites.some((s) => s.includes("Successfully upgraded"))).toBe(true);
+  });
+});