feat: add OCI Annotations constants

[wangxiaoxuan273]

What this PR does / why we need it

Adds standard OCI annotation key constants per the OCI Image Spec v1.1.1 annotations.

Includes a test file that validates all constant values.

Which issue(s) this PR resolves / fixes

Part of the File Store implementation split (issue 1 of 10).
Related: #328, #37

Please check the following list

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.76%. Comparing base (5953fb1) to head (7e54d61).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #351   +/-   ##
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  Coverage   91.76%   91.76%           
=======================================
  Files          64       64           
  Lines        2755     2755           
  Branches      364      364           
=======================================
  Hits         2528     2528           
  Misses        138      138           
  Partials       89       89           

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[Copilot]

Pull request overview

Adds a new public constants surface for standard OCI image annotation keys (per OCI Image Spec v1.1.1), along with unit tests to prevent accidental value drift. This fits into the OrasProject.Oras.Oci model/constants area similarly to existing Oci.MediaType.

Changes:

• Introduces OrasProject.Oras.Oci.Annotations with constants for standard OCI annotation keys.
• Adds AnnotationsTest to assert all constants match the spec-defined strings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/OrasProject.Oras/Oci/Annotations.cs	New public constants class exposing OCI annotation keys with XML docs + spec link.
tests/OrasProject.Oras.Tests/Oci/AnnotationsTest.cs	Unit test validating all annotation constant values.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[akashsinghal]

Caution

This review was generated by Copilot using Claude Opus 4.6, mimicking the review style of shizhMSFT. This is not the real shizhMSFT.

Review of PR #351

Verified all 14 constants match OCI Image Spec v1.1.1 annotations.md and specs-go/v1/annotations.go exactly. Clean, spec-compliant, well-tested. Three observations:

1. Url should be URL (Annotations.cs:37)

The Go spec uses AnnotationURL (uppercase acronym). The .NET naming convention for acronyms of 2 letters is all-caps: IO, UI, DB — and URL. See .NET naming guidelines for acronyms. Url → URL. This is a public API name; fix it before it ships.

2. XML docs should link spec sections directly

The class-level doc links to the annotations page, which is good. But individual constant docs could benefit from noting which spec section defines them — especially RefName which has specific character-set and grammar constraints (alphanum ("/" component)*). Developers will not read your code but read your documentation before invoking the API. Consider adding at minimum a note on RefName's character-set constraint and on Created's RFC 3339 format requirement.

3. nit: Test asserts each value individually but could use [Theory] / [InlineData]

14 assertions in a single [Fact] method. If assertion #3 fails, assertions #4-14 don't run. A [Theory] with [InlineData] would give per-constant failure granularity. Same coverage, better diagnostics:

[Theory]
[InlineData(nameof(Annotations.Created), "org.opencontainers.image.created")]
[InlineData(nameof(Annotations.Authors), "org.opencontainers.image.authors")]
// ...
public void OciAnnotation_HasCorrectValue(string name, string expected)
{
    var field = typeof(Annotations).GetField(name);
    Assert.Equal(expected, field!.GetValue(null));
}

Otherwise SGTM.