fix(copy): re-push referenced manifests when the destination reports them missing

[Orexii]

What this does

Makes CopyGraph self-heal a partially-copied graph. When the destination rejects a manifest push because it is missing referenced content (MANIFEST_BLOB_UNKNOWN / BLOB_UNKNOWN / MANIFEST_UNKNOWN), the copy now re-pushes the node's manifest successors and retries the push once, instead of returning the error.

Why

CopyGraph skips a node whose sub-DAG it believes already exists ("skip if a rooted sub-DAG exists"). If a prior copy was interrupted after a child manifest's blobs were uploaded but before the child manifest was committed, the destination can report the child as present (Exists/HEAD) yet reject the parent that references it. For a multi-arch image index this means the index push fails with MANIFEST_BLOB_UNKNOWN, and since the child is skipped on every run, re-running the same oras cp --recursive never recovers.

This was hit in production mirroring a multi-arch image (mongo:7.0.34) between two registries; the only manual workaround was copying each child manifest by digest and re-pushing the index.

How

• copyGraph: wrap the node push; on a "missing referenced content" error, re-copy the manifest successors unconditionally (bypassing the existence short-circuit) and retry the push once. No-op on the success path; blob successors are not re-pushed.
• isMissingReferencedContentError: typed classification via errcode (handles both a single errcode.Error and a multi-error errcode.Errors); no string matching.

Testing

• TestCopyGraph_SelfHealsInterruptedIndexCopy: end-to-end reproduction against a destination that emulates the interrupted state — fails without this change (MANIFEST_BLOB_UNKNOWN), passes with it.
• TestIsMissingReferencedContentError: unit test for the classifier (100% covered).
• go test -race ./... passes; gofmt / go vet clean; new files carry the Apache license header.

Notes for reviewers

• Design choice: detection currently uses the registry-specific errcode package from transport-agnostic copyGraph. An alternative is a sentinel in errdef that the remote manifestStore maps MANIFEST_BLOB_UNKNOWN / BLOB_UNKNOWN onto. Happy to switch if you prefer that layering.
• Scope: recovers the immediate manifest successors (the index -> child-manifest case). No API or signature changes; no breaking changes.

Fixes #1211

[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.09%. Comparing base (2d37097) to head (307ce2c).

Files with missing lines	Patch %	Lines
copy.go	86.20%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1212   +/-   ##
=======================================
  Coverage   83.09%   83.09%           
=======================================
  Files          82       82           
  Lines        5808     5834   +26     
=======================================
+ Hits         4826     4848   +22     
- Misses        603      605    +2     
- Partials      379      381    +2     

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[TerryHowe]

/lgtm

[Orexii]

@TerryHowe anything needed to do on my end for this? Please let me know. Thanks