fix(build): retry transient sitemap endpoint fetches

The TypeScript helper refactor exposed a separate production risk: builds can fail when the sitemap-endpoints API returns a transient 5xx during Pagefind index generation. Move that fetch into a small helper with retry logic and add a focused prebuild test so this failure mode is caught without waiting on a full deploy.

Constraint: Must keep Pagefind index generation behavior the same when the endpoint is healthy
Rejected: Ignore the failure as external-only | transient 5xx responses can still block deploys and CI repeatedly
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When external build inputs can fail transiently, add a small retryable helper plus a direct test before relying on full deploy verification
Tested: pnpm exec vitest run test/prebuild/load-sitemap-endpoints.test.ts; pnpm run typecheck; pnpm run test-prebuild
Not-tested: Full production redeploy completion before push

test/prebuild/load-sitemap-endpoints.test.ts

@@ -0,0 +1,94 @@
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+    vi
+} from 'vitest'
+
+import axios from 'axios'
+
+import {
+    fetchJsonWithRetries,
+    shouldRetryError
+} from '~/helpers/pagefind/load-sitemap-endpoints'
+
+vi.mock( 'axios', () => {
+    return {
+        default: {
+            get: vi.fn()
+        }
+    }
+} )
+
+describe( 'load sitemap endpoints helper', () => {
+    beforeEach( () => {
+        vi.mocked( axios.get ).mockReset()
+    } )
+
+    it( 'retries transient 5xx errors and eventually resolves', async () => {
+        const axiosGet = vi.mocked( axios.get )
+
+        axiosGet
+            .mockRejectedValueOnce({
+                response: {
+                    status: 502
+                }
+            })
+            .mockResolvedValueOnce({
+                data: {
+                    ok: true
+                }
+            } )
+
+        const data = await fetchJsonWithRetries( 'https://api.doesitarm.com/sitemap-endpoints.json', {
+            attempts: 2,
+            delayMs: 0
+        } )
+
+        expect( data ).toEqual({
+            ok: true
+        } )
+        expect( axiosGet ).toHaveBeenCalledTimes( 2 )
+    } )
+
+    it( 'does not retry non-5xx errors', async () => {
+        const axiosGet = vi.mocked( axios.get )
+
+        axiosGet.mockRejectedValueOnce({
+            response: {
+                status: 404
+            }
+        })
+
+        await expect( fetchJsonWithRetries( 'https://api.doesitarm.com/sitemap-endpoints.json', {
+            attempts: 3,
+            delayMs: 0
+        } ) ).rejects.toEqual({
+            response: {
+                status: 404
+            }
+        })
+
+        expect( axiosGet ).toHaveBeenCalledTimes( 1 )
+    } )
+
+    it( 'classifies retryable server errors', () => {
+        expect( shouldRetryError( {
+            response: {
+                status: 502
+            }
+        } ) ).toBe( true )
+        expect( shouldRetryError( {
+            response: {
+                status: 503
+            }
+        } ) ).toBe( true )
+        expect( shouldRetryError( {
+            response: {
+                status: 404
+            }
+        } ) ).toBe( false )
+        expect( shouldRetryError( new Error( 'network' ) ) ).toBe( false )
+    } )
+} )