reject trailing newline in host label validators

[metsw24-max]

botocore's host label validators all anchor with $, which in Python also matches just before a trailing newline, so a value such as us-east-1\n, mybucket\n or a data\n host prefix passes validation and the unchanged string is then used to build the request host. validate_region_name even documents that its argument must be a valid host label, yet it accepts the newline, and the same gap exists in validate_bucket_name, the HeaderToHostHoister hostname check, the S3ControlEndpointSetter host label check and the serialiser host prefix check. The region case is the most exposed because that value can be sourced from an S3 cross-region redirect response and then feeds endpoint resolution and the SigV4 signing scope. is_valid_endpoint_url already screens for \t\r\n through UNSAFE_URL_CHARS, so these label validators are the remaining place a control character can slip past. I moved the trailing anchor to \Z in each one so a match has to reach the actual end of the string, and left the ARN and resource regexes alone since those re-extract a clean label and never carry a newline through. Valid regions, bucket names and host labels behave exactly as before.

[metsw24-max]

gentle ping