cmd/cue: byte values do not roundtrip with JSON as the encoder is not schema-aware

[mvdan]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.13.0-alpha.1.0.20250314174817-9e333c606194

go version devel go1.25-21417518a9 2025-03-15 06:05:16 -0700
      -buildmode exe
       -compiler gc
  DefaultGODEBUG decoratemappings=0,gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v3
             vcs git
    vcs.revision 9e333c606194edc184ffd518be71509a4b52bc1c
        vcs.time 2025-03-14T17:48:17Z
    vcs.modified false
cue.lang.version v0.13.0

Does this issue reproduce with the latest stable release?

Yes.

What did you do?

exec cue export input.cue
cmp stdout export.json

exec cue vet -c input.cue export.json

-- input.cue --
foo: '\x00\x01\x02'
-- export.json --
{
    "foo": "AAEC"
}

What did you expect to see?

This should succeed; exporting bytes to JSON results in a base64-encoded string, and I should be able to validate the data with the original CUE as a concrete schema.

What did you see instead?

> exec cue export input.cue
[stdout]
{
    "foo": "AAEC"
}
> cmp stdout export.json
> exec cue vet -c input.cue export.json
[stderr]
foo: conflicting values "AAEC" and '\x00\x01\x02' (mismatched types string and bytes):
    ./export.json:2:12
    ./input.cue:1:6
[exit status 1]

I understand why this happens; our JSON decoder is currently not schema-aware, so it defaults to decoding the string "AAEC" as a CUE string as-is, rather than noticing that the schema expects bytes, meaning that it should do a base64 decode of the contents and end up with the CUE value '\x00\x01\x02'.