services/stellar-archivist: fix nil pointer panics in S3Storage.ListFiles

[satyamz]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

services/stellar-archivist: fix nil pointer panics in S3Storage.ListFiles

Why

The pagination goroutine in ListFiles dereferences resp.IsTruncated and c.Key without nil checks. Both are pointer fields from aws-sdk-go that can legitimately be nil — some S3-compatible backends (MinIO, GCS interop) omit IsTruncated entirely. Because the deref happens in a goroutine, a nil value panics the whole process instead of returning an error.

panic: runtime error: invalid memory address or nil pointer dereference
github.com/stellar/go-stellar-sdk/support/storage.(*S3Storage).ListFiles.func1()
    support/storage/s3.go:232

Known limitations

[TODO or N/A]

[Copilot]

Pull request overview

Fixes a crash in support/storage’s S3 backend by making S3Storage.ListFiles resilient to nil pointer fields returned by some S3-compatible APIs during pagination.

Changes:

• Avoid direct dereference of *c.Key by using aws.StringValue(c.Key) when logging/sending keys.
• Avoid direct dereference of *resp.IsTruncated by using aws.BoolValue(resp.IsTruncated) to drive pagination.

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[SirTyson]

LGTM