S3 Express endpoints do not support dualstack variants

[jiuker]

AWS documents dualstack variants for every S3 Express endpoint, both regional and zonal (Endpoint directory buckets, Networking for directory buckets in an AZ):

• Regional: s3express-control-dualstack.<region>.amazonaws.com
• Zonal: s3express-<zone-id>.dualstack.<region>.amazonaws.com (e.g. s3express-use1-az4.dualstack.us-east-1.amazonaws.com)

minio-go currently only ever produces the non-dualstack forms:

• awsS3ExpressEndpointMap (endpoints.go) holds the plain s3express-control.<region>.amazonaws.com entries,
• getS3ExpressEndpoint derives zonal endpoints as s3express-<zone-id>.<region>.amazonaws.com.

Unlike the regular S3 path (getS3Endpoint honors useDualstack via Client.s3DualstackEnabled, which New() enables by default for Amazon S3), the S3 Express path ignores the dualstack flag entirely — SetS3EnableDualstack and Options.S3Dualstack have no effect on S3 Express requests. Clients on IPv6-only networks or that otherwise need the dualstack endpoints cannot get them.

Suggested fix: thread the dualstack flag into getS3ExpressEndpoint the same way getS3Endpoint does, producing:

• s3express-control-dualstack.<region>.amazonaws.com for the regional endpoint,
• s3express-<zone-id>.dualstack.<region>.amazonaws.com for the zonal endpoint.

Related: #2293 (S3 Express endpoint selection work).

[jiuker]

Additional detail from a regex audit (related to this issue): the dualstack variants are also not recognized by the host-name regexes in pkg/s3utils/utils.go, so and fail on them too:

• s3express-control-dualstack.us-east-1.amazonaws.com → region extraction returns ""
• s3express-use1-az4.dualstack.us-east-1.amazonaws.com → ""
• s3express-usw2-lax1-az1.dualstack.us-west-2.amazonaws.com (Local Zone) → ""

Neither amazonS3HostExpress nor amazonS3HostExpressControl matches the dualstack forms, and the amazonS3HostDot fallback deliberately returns "" for any s3express- host (the "xpress-" guard). Any fix for the endpoint-selection side (PR #2293) should extend these regexes in the same change, or this stays half-broken: the client would pick the dualstack host but the SDK would refuse to treat it as an Amazon endpoint.