feat: support artifact uploads for OCI container packages (#8070)

# Fix OCI artifact uploads with`oras`

## Problem

ORAS (OCI Registry As Storage) artifact uploads were failing with several HTTP-related errors when pushing to Forgejo's container registry. This prevented users from storing OCI artifacts like `artifacthub-repo.yaml` in commands like `oras push [...] artifacthub-repo.yaml:application/vnd.cncf.artifacthub.repository-metadata.layer.v1.yaml`.

This has been discussed previously in https://github.com/go-gitea/gitea/issues/25846

## Root Causes and Fixes

### 1. Missing Content-Length for Empty Blobs

**Issue**: Empty blobs (size 0) were not getting the required `Content-Length: 0` header, causing ORAS to fail with "unknown response Content-Length".

**Fix**: Changed the condition in `setResponseHeaders` from `if h.ContentLength != 0` to `if h.ContentLength >= 0` to ensure the Content-Length header is always set for valid blob sizes.

```go
// Before
if h.ContentLength != 0 {
    resp.Header().Set("Content-Length", strconv.FormatInt(h.ContentLength, 10))
}

// After
if h.ContentLength >= 0 {
    resp.Header().Set("Content-Length", strconv.FormatInt(h.ContentLength, 10))
}
```

### 2. Content-Length Mismatch in JSON Error Responses

**Issue**: The `jsonResponse` function was calling `WriteHeader()` before writing JSON content, causing "wrote more than the declared Content-Length" errors when the HTTP stack calculated a different Content-Length than what was actually written.

**Fix**: Modified `jsonResponse` to buffer JSON content first, calculate the exact Content-Length, then write the complete response.

### 3. Incomplete HTTP Responses in Error Handling

**Issue**: The `apiError` function was only setting response headers without writing any response body, causing EOF errors when clients expected a complete HTTP response.

**Fix**: Updated `apiError` to write proper JSON error responses following the OCI Distribution Specification format with `code` and `message` fields.

### 4. Empty Config Blob Handling for OCI Artifacts

**Issue**: OCI artifacts often have empty config blobs (required by spec but contain no data). The JSON decoder was failing with EOF when trying to parse these empty configs.

**Fix**: Added EOF handling in `parseOCIImageConfig` to return a valid default metadata object for empty config blobs.

```go
if err := json.NewDecoder(r).Decode(&image); err != nil {
    // Handle empty config blobs (common in OCI artifacts)
    if err == io.EOF {
        return &Metadata{
            Type:     TypeOCI,
            Platform: DefaultPlatform,
        }, nil
    }
    return nil, err
}
```

## Testing

Verified that ORAS artifact uploads now work correctly:

```bash
oras push registry/owner/package:artifacthub.io \
  --config /dev/null:application/vnd.cncf.artifacthub.config.v1+yaml \
  artifacthub-repo.yaml:application/vnd.cncf.artifacthub.repository-metadata.layer.v1.yaml
```

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8070
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

modules/packages/container/metadata_test.go

@@ -4,6 +4,7 @@
 package container
 
 import (
+	"io"
 	"strings"
 	"testing"
 
@@ -60,3 +61,49 @@ func TestParseImageConfig(t *testing.T) {
 	assert.Equal(t, projectURL, metadata.ProjectURL)
 	assert.Equal(t, repositoryURL, metadata.RepositoryURL)
 }
+
+func TestParseImageConfigEmptyBlob(t *testing.T) {
+	t.Run("Empty config blob (EOF)", func(t *testing.T) {
+		// Test empty reader (simulates empty config blob common in OCI artifacts)
+		metadata, err := ParseImageConfig(oci.MediaTypeImageManifest, strings.NewReader(""))
+		require.NoError(t, err)
+
+		assert.Equal(t, TypeOCI, metadata.Type)
+		assert.Equal(t, DefaultPlatform, metadata.Platform)
+		assert.Empty(t, metadata.Description)
+		assert.Empty(t, metadata.Authors)
+		assert.Empty(t, metadata.Labels)
+		assert.Empty(t, metadata.Manifests)
+	})
+
+	t.Run("Empty JSON object", func(t *testing.T) {
+		// Test minimal valid JSON config
+		metadata, err := ParseImageConfig(oci.MediaTypeImageManifest, strings.NewReader("{}"))
+		require.NoError(t, err)
+
+		assert.Equal(t, TypeOCI, metadata.Type)
+		assert.Equal(t, DefaultPlatform, metadata.Platform)
+		assert.Empty(t, metadata.Description)
+		assert.Empty(t, metadata.Authors)
+	})
+
+	t.Run("Invalid JSON still returns error", func(t *testing.T) {
+		// Test that actual JSON errors (not EOF) are still returned
+		_, err := ParseImageConfig(oci.MediaTypeImageManifest, strings.NewReader("{invalid json"))
+		require.Error(t, err)
+		assert.NotEqual(t, io.EOF, err)
+	})
+
+	t.Run("OCI artifact with empty config", func(t *testing.T) {
+		// Test OCI artifact scenario with minimal config
+		configOCI := `{"config": {}}`
+		metadata, err := ParseImageConfig(oci.MediaTypeImageManifest, strings.NewReader(configOCI))
+		require.NoError(t, err)
+
+		assert.Equal(t, TypeOCI, metadata.Type)
+		assert.Equal(t, DefaultPlatform, metadata.Platform)
+		assert.Empty(t, metadata.Description)
+		assert.Empty(t, metadata.Authors)
+		assert.Empty(t, metadata.ImageLayers)
+	})
+}