feat: Add PublicApiAttribute for marking PublicApi test definitions

[Copilot]

Implements PublicApiAttribute across all test frameworks (TUnit, XUnit.V3, MSTest, NUnit) to enable consistent marking of PublicApi definitions in test projects, mirroring the existing FunctionalTestAttribute pattern.

Changes

• Attribute implementation: Added PublicApiAttribute to each framework, inheriting from the appropriate base (CategoryTraitBaseAttribute, TestTraitBaseAttribute, or CategoryAttribute)
• Internal constants: Added PublicApi constant to Internals.cs in each framework
• Test coverage: Created unit tests and snapshot verifications for all .NET versions (6.0, 8.0, 9.0, 10.0)
• Public API surface: Updated API snapshots to include the new attribute

Usage

[PublicApi]
public class MyPublicApiTests
{
    [Test]
    [PublicApi]
    public void ValidatePublicApiContract()
    {
        // Test implementation
    }
}

The attribute follows the same semantics as other test categorization attributes: applicable to Assembly, Class, or Method with AllowMultiple = true.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>feature: PublicApiAttribute</issue_title>
<issue_description>### Prerequisites

• I have searched for similar feature requests in both open and closed issues
• I have read the documentation to confirm this feature doesn't already exist

Feature Description

Implement in each library an PublicApiAttribute similar to FunctionalTestAttribute

Use Case

To easy mark PublicApi Definitions in Test Projects

Proposed Solution

No response

Alternatives Considered

No response

Additional Context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feature: PublicApiAttribute #966

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[samtrion]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.82%. Comparing base (f131703) to head (54ac374).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #967      +/-   ##
==========================================
+ Coverage   83.54%   83.82%   +0.28%     
==========================================
  Files          84       88       +4     
  Lines         231      235       +4     
  Branches       17       17              
==========================================
+ Hits          193      197       +4     
  Misses         32       32              
  Partials        6        6              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.