Tolerate transient manifest timeouts (#1016)

Transient timeout fetching manifests have increased significantly
recently, especially with private runners.

```
Fetching manifest data from https://raw.githubusercontent.com/astral-sh/versions/main/v1/uv.ndjson ...
Error: The operation was aborted due to timeout
```

Retry transient manifest network failures up to three times with a
progressive backoff (not exponential), keeping the total wait bounded
while making setup resilient to short network blips.

Co-authored-by: Raymond <arguile-@users.noreply.github.com>

__tests__/download/manifest.test.ts

@@ -1,4 +1,11 @@
-import { beforeEach, describe, expect, it, jest } from "@jest/globals";
+import {
+  afterEach,
+  beforeEach,
+  describe,
+  expect,
+  it,
+  jest,
+} from "@jest/globals";
 
 // biome-ignore lint/suspicious/noExplicitAny: Mock requires flexible typing in tests.
 const mockFetch = jest.fn<any>();
@@ -13,6 +20,7 @@ jest.unstable_mockModule("../../src/utils/fetch", () => ({
 }));
 
 const {
+  MANIFEST_FETCH_ATTEMPTS,
   clearManifestCache,
   fetchManifest,
   getAllVersions,
@@ -73,6 +81,10 @@ describe("manifest", () => {
     mockFetch.mockReset();
   });
 
+  afterEach(() => {
+    jest.useRealTimers();
+  });
+
   describe("fetchManifest", () => {
     it("fetches and parses manifest data", async () => {
       mockFetch.mockResolvedValue(
@@ -86,6 +98,34 @@ describe("manifest", () => {
       expect(versions[1]?.version).toBe("0.9.25");
     });
 
+    it("retries network failures", async () => {
+      jest.useFakeTimers();
+      mockFetch
+        .mockRejectedValueOnce(new Error("request timed out"))
+        .mockResolvedValueOnce(
+          createMockResponse(true, 200, "OK", sampleManifestResponse),
+        );
+
+      const result = fetchManifest();
+      await jest.runAllTimersAsync();
+
+      await expect(result).resolves.toHaveLength(2);
+      expect(mockFetch).toHaveBeenCalledTimes(2);
+    });
+
+    it("stops after the configured number of network failures", async () => {
+      jest.useFakeTimers();
+      mockFetch.mockRejectedValue(new Error("request timed out"));
+
+      const result = expect(fetchManifest()).rejects.toThrow(
+        "request timed out",
+      );
+      await jest.runAllTimersAsync();
+
+      await result;
+      expect(mockFetch).toHaveBeenCalledTimes(MANIFEST_FETCH_ATTEMPTS);
+    });
+
     it("throws on a failed fetch", async () => {
       mockFetch.mockResolvedValue(
         createMockResponse(false, 500, "Internal Server Error", ""),