feat: change foreign layer error message at repository.Fetch()

[wangxiaoxuan273]

Resolves #319

Current result with oras-cli:

$ oras copy docker.io/library/hello-world@sha256:314cc0309465c7be2936df274b258c843f5a49eb21f63b719d60b51564070b8f wxxacr.azurecr.io/temp439:test00

$ Error: the artifact with foreign layer sha256:5ead999142ecce15e02523c49706a633fa708374d94bb65a254e3a3c117d609b is not supported: sha256:5ead999142ecce15e02523c49706a633fa708374d94bb65a254e3a3c117d609b: not found

Signed-off-by: wangxiaoxuan273 wangxiaoxuan119@gmail.com

[codecov-commenter]

Codecov Report

Merging #377 (617f44f) into main (ff05195) will increase coverage by 1.59%.
The diff coverage is 34.48%.

❗ Current head 617f44f differs from pull request most recent head dd68802. Consider uploading reports for the commit dd68802 to get more accurate results

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   72.24%   73.84%   +1.59%     
==========================================
  Files          42       45       +3     
  Lines        4093     4339     +246     
==========================================
+ Hits         2957     3204     +247     
+ Misses        855      844      -11     
- Partials      281      291      +10     

Impacted Files	Coverage Δ
internal/syncutil/limitgroup.go	0.00% <0.00%> (ø)
registry/remote/repository.go	76.15% <53.84%> (+7.56%)	⬆️
content.go	74.55% <100.00%> (-1.20%)	⬇️
content/oci/readonlystorage.go	93.93% <0.00%> (ø)
content/oci/readonlyoci.go	66.12% <0.00%> (ø)
content/oci/storage.go	64.95% <0.00%> (+2.85%)	⬆️

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[qweeah]

Some repositories may contain foreign layers which pointing to the repository itself, e.g.

> oras manifest fetch mcr.microsoft.com/windows/servercore:1607-KB5019964 --pretty --platform windows/amd64
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.docker.container.image.v1+json",
    "size": 787,
    "digest": "sha256:f443e6ff891920258e9c0ce66f7f5472793b26691ea18d7fc87af0472c677430"
  },
  "layers": [
    {
      "mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
      "size": 4069985900,
      "digest": "sha256:3889bb8d808bbae6fa5a33e07093e65c31371bcf9e4c38c21be6b9af52ad1548",
      "urls": [
        "https://mcr.microsoft.com/v2/windows/servercore/blobs/sha256:3889bb8d808bbae6fa5a33e07093e65c31371bcf9e4c38c21be6b9af52ad1548"
      ]
    },
    {
      "mediaType": "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip",
      "size": 2213621320,
      "digest": "sha256:29226bae1723379e75abf7d7da45bbf67d5d730611c58bf86df104245db30204",
      "urls": [
        "https://mcr.microsoft.com/v2/windows/servercore/blobs/sha256:29226bae1723379e75abf7d7da45bbf67d5d730611c58bf86df104245db30204"
      ]
    }
  ]
}

So it's better to return unsupported only when a layer is foreign and not found. Same logic should apply to OCI target.

[shizhMSFT]

LGTM

[Wwwsylvia]

LGTM