test(io): cover an oss:// round trip against the S3 test endpoint

[plusplusjiajia]

oss:// support is already on main — ResolveFileIOName routes it, CanonicalizeS3Scheme normalizes it, IsS3FileIOCredentialPrefix accepts it — but nothing exercises it against a live endpoint. This adds that evidence rather than asserting it.

The test writes, reads back and deletes an object addressed as oss://, through the path a REST catalog takes: resolution picks the S3 FileIO, and the vended credential — scoped to s3, while the data is addressed as oss:// — is matched against the canonicalized location.

No new infrastructure: it runs against ICEBERG_TEST_S3_URI, which in CI is the MinIO the AWS jobs already start, and skips when unset like the other S3 integration tests. The scheme is only a routing label; the connection comes from s3.endpoint

[wgtmac]

How about still removing oss support from #889 and then you add it back by adding test and documentation to fileio.md to instruct users how to config for oss?

[plusplusjiajia]

How about still removing oss support from #889 and then you add it back by adding test and documentation to fileio.md to instruct users how to config for oss?

@wgtmac Works for me — I'll keep this PR and grow it into that: once #889 lands I'll rebase, restore the scheme, and add fileio.md covering how to configure it.