Function normalise_region 

fn normalise_region(raw: Option<&str>) -> String

Normalise an S3 LocationConstraint string to a canonical AWS region name.

AWS quirks handled here:

• None / empty string → "us-east-1" (us-east-1 buckets return no constraint)
• "EU" → "eu-west-1" (legacy alias from before region-specific EU endpoints)

All other values are returned as-is.