fix(checksums): reject md5 instead of silently returning crc32 (#4513)

The UnknownChecksumAlgorithmError message omitted crc64nvme and advertised
the deprecated md5. Parsing "md5" also returned a Crc32 algorithm and its
into_impl produced a 4-byte CRC32, so a caller asking for MD5 silently got
the wrong digest.

Enumerate the accepted algorithms (crc32, crc32c, crc64nvme, sha1, sha256)
in the error message, remove the unused Md5 enum variant, and make parsing
"md5" fail loudly. Add tests covering the message contents and the md5
parse error.

crates/checksums/src/error.rs

@@ -36,7 +36,7 @@ impl fmt::Display for UnknownChecksumAlgorithmError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
             f,
-            r#"unknown checksum algorithm "{}", please pass a known algorithm name ("crc32", "crc32c", "sha1", "sha256", "md5")"#,
+            r#"unknown checksum algorithm "{}", please pass a known algorithm name ("crc32", "crc32c", "crc64nvme", "sha1", "sha256")"#,
             self.checksum_algorithm
         )
     }