Add S3 integration tests

[PeterDowdy]

This PR adds S3 integration tests through minIO as a mocked S3 provider. It covers most of the happy and sad paths in the project, with moderate rigour. It doesn't check transient errors (since the library just dies on them anyways), TLS verification (since that seems like it's drifting out of testing S3), or very large synthetic files (to keep runtime short).

The minIO test depends on an env var so developers without minIO can safely bypass it.

This effort surfaced a few fairly small gaps in S3 file-handling that could be filled:

• No retries (as mentioned)
• A single failure in multipart aborts the whole multipart operation (related)
• SigV2 write unimplemented (seems pretty legacy though)
• CRAM UR: tag can't reach S3
• CRAM reference caching only ever uses local disk (not so much a gap as a possible useful feature)
• bgzip CLI can't write to S3
• Loading a .crai always downloads it to disk

Assisted-by: Claude:claude-sonnet-5

[whitwham]

Thank you for this. I won't have time to look at it for the next few weeks but I will get round to it.

[PeterDowdy]

Thank you for this. I won't have time to look at it for the next few weeks but I will get round to it.

No rush!