types/model: restrict digest hash part to a minimum of 2 characters (#3858)

This allows users of a valid Digest to know it has a minimum of 2
characters in the hash part for use when sharding.

This is a reasonable restriction as the hash part is a SHA256 hash which
is 64 characters long, which is the common hash used. There is no
anticipation of using a hash with less than 2 characters.

Also, add MustParseDigest.

Also, replace Digest.Type with Digest.Split for getting both the type
and hash parts together, which is most the common case when asking for
either.

types/model/name.go

@@ -7,6 +7,7 @@ import (
 	"hash/maphash"
 	"io"
 	"log/slog"
+	"path"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -589,10 +590,20 @@ func ParseNameFromURLPath(s, fill string) Name {
 // Example:
 //
 //	ParseName("example.com/namespace/model:tag+build").URLPath() // returns "/example.com/namespace/model:tag"
-func (r Name) URLPath() string {
+func (r Name) DisplayURLPath() string {
 	return r.DisplayShortest(MaskNothing)
 }
 
+// URLPath returns a complete, canonicalized, relative URL path using the parts of a
+// complete Name in the form:
+//
+//	<host>/<namespace>/<model>/<tag>
+//
+// The parts are downcased.
+func (r Name) URLPath() string {
+	return strings.ToLower(path.Join(r.parts[:PartBuild]...))
+}
+
 // ParseNameFromFilepath parses a file path into a Name. The input string must be a
 // valid file path representation of a model name in the form:
 //