* s3: use url.PathUnescape for X-Amz-Copy-Source header (#8544)
The X-Amz-Copy-Source header is a URL-encoded path, not a query string.
Using url.QueryUnescape incorrectly converts literal '+' characters to
spaces, which can cause object key mismatches during copy operations.
Switch to url.PathUnescape in CopyObjectHandler, CopyObjectPartHandler,
and pathToBucketObjectAndVersion to correctly handle special characters
like '!', '+', and other RFC 3986 sub-delimiters that S3 clients may
percent-encode (e.g. '!' as %21).
* s3: add path validation to CopyObjectPartHandler
CopyObjectPartHandler was missing the validateTableBucketObjectPath
checks that CopyObjectHandler has, allowing potential path traversal
in the source bucket/object of copy part requests.
* s3: fix case-sensitive HeadersRegexp for copy source routing
The HeadersRegexp for X-Amz-Copy-Source used `%2F` which only matched
uppercase hex encoding. RFC 3986 allows both `%2F` and `%2f`, so
clients sending lowercase percent-encoding would bypass the copy
handler and hit PutObjectHandler instead. Add (?i) flag for
case-insensitive matching.
Also add test coverage for the versionId branch in
pathToBucketObjectAndVersion and for lowercase %2f routing.
