Spike/federation test endpoint

[alukach]

What I'm changing

How I did it

How to test it

PR Checklist

• This PR has no breaking changes.
• I have updated or added new tests to cover the changes in this PR.
• This PR affects the Source Cooperative Frontend & API,
  and I have opened issue/PR #XXX to track the change.

Related Issues

[github-actions]

🚀 Latest commit deployed to https://source-data-proxy-pr-145.source-coop.workers.dev

• Date: 2026-06-03T03:56:21Z
• Commit: b6184a6

[alukach]

Confirmed working:

▶ curl --silent https://source-data-proxy-pr-145.source-coop.workers.dev/_test | jq
{
  "config": {
    "audience": "source-coop-data-proxy",
    "bucket": "alukach-demo-bucket",
    "discovery_url": "https://source-data-proxy-pr-145.source-coop.workers.dev/.well-known/openid-configuration",
    "issuer_used": "https://source-data-proxy-pr-145.source-coop.workers.dev",
    "region": "us-west-2",
    "role_arn": "arn:aws:iam::470592060578:role/test-data-source-coop-oidc",
    "subject": "scv1:conn:test:federation-spike"
  },
  "mint_token": {
    "ok": true
  },
  "s3_list": {
    "keys": [
      "aschild_verify.txt"
    ],
    "ok": true,
    "raw": "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<ListBucketResult xmlns=\"http://s3.amazonaws.com/doc/2006-03-01/\"><Name>alukach-demo-bucket</Name><Prefix></Prefix><KeyCount>1</KeyCount><MaxKeys>100</MaxKeys><IsTruncated>false</IsTruncated><Contents><Key>aschild_verify.txt</Key><LastModified>2026-06-03T03:58:26.000Z</LastModified><ETag>&quot;b40c8a7d89ca5c2cdd026e04beacf043&quot;</ETag><ChecksumAlgorithm>CRC64NVME</ChecksumAlgorithm><ChecksumType>FULL_OBJECT</ChecksumType><Size>913</Size><StorageClass>STANDARD</StorageClass></Contents></ListBucketResult>",
    "status": 200
  },
  "sts": {
    "access_key_id": "ASIAW3ELY7SRPSG2R74Y",
    "expiration": "2026-06-03T04:22:37Z",
    "ok": true
  }
}