XUnit v3 upgrade + fixes

[perkops]

Summary

Upgrade and modernization of Atc.Test to xUnit v3 with improved data attribute capabilities, deterministic frozen value handling, stronger safety around specimen reuse, and CI pipeline upgrade to .NET 9.

Key Changes

Framework & Infrastructure

• Migrated custom data attributes to xUnit v3 async extensibility (ValueTask<IReadOnlyCollection<ITheoryDataRow>>).
• Library depends on xunit.v3.extensibility.core; consumers must add a direct PackageReference to xunit.v3.

Data Attribute Enhancements

MemberAutoNSubstituteDataAttribute

• Two-phase frozen handling: (1) positional reuse (2) exact-type promotion of earlier supplied values.
• Promotion restricted to exact declared type only (no interface/base widening).

ClassAutoNSubstituteDataAttribute

• Positional-only reuse (no promotion) for explicitness.
• Introduced FrozenParameterInjector (shared helper extracting and unifying frozen injection logic).

Frozen Semantics Hardening

• Negative test preventing accidental cross-interface promotion (dual-interface specimen).
• Replaced broad IsInstanceOfType matching with exact declared type + earliest index rule.
• Documented scenarios: positional reuse, exact-type promotion, explicit non-promotion across interfaces.

Documentation & Developer Experience

• Expanded XML docs (resolution order, promotion rules, fixture isolation).
• Restructured README (Introduction, Why Atc.Test, Features, Getting Started, Advanced Usage, Requirements).
• Added explicit xUnit v3 only compatibility section and optional guard snippet.
• Explained rationale for not making xunit.v3 a transitive dependency.
• Added a value proposition section (“Why Atc.Test”).

Tests

• Added coverage for:
  • Positional frozen reuse
  • Earlier-supplied exact-type promotion
  • Negative non-promotion across interfaces
• Reflection-based tests verify display name handling (where relevant) and metadata preservation for class data rows.

Refactoring & Cleanup

• Centralized frozen logic in FrozenParameterInjector.
• Reduced duplication between class and member data attributes.
• Simplified display name handling (removed visual markers).
• Ensured constructor argument exposure
• Clarified separation of responsibilities (fixture creation vs injection pipeline).

CI / Pipeline Modernization

• Updated pre-integration, post-integration, and release GitHub workflows to use .NET 9 (actions/setup-dotnet 9.0.x).

Breaking Changes

Area	Change	Impact	Action Required
xUnit Version	Requires xUnit v3	xUnit v2 projects will not compile	Add PackageReference Include="xunit.v3"
Member Data Frozen Behavior	Exact-type earlier supplied value promotion	Some tests may now see reuse where a new instance was previously generated	Remove [Frozen] or supply explicit alternate instance if reuse undesired
Assignable-Type Promotion	Interface/base promotion removed	May reveal hidden coupling or reliance on broad matching	Adjust tests to explicitly freeze intended type
No Transitive xUnit	xunit.v3 not pulled in indirectly	Missing explicit reference causes compile errors	Add explicit reference

Rationale

• xUnit v3 provides cleaner async-aware data extensibility and richer row metadata.
• Exact-type promotion increases ergonomics for member data while remaining predictable.
• Negative tests codify design boundaries and prevent silent regression.
• Central helper (FrozenParameterInjector) simplifies maintenance and future enhancement.
• SDK pin ensures reproducible builds across contributors and CI.

Implementation Notes

Concern	Approach
Promotion Selection	Earliest supplied argument whose declared type exactly matches the [Frozen] parameter type
Member vs Class Data	Promotion enabled only for member data; class data stays positional
Fixture Isolation	New fixture instance per data row
Metadata Preservation	ITheoryDataRow properties copied (Label, Explicit, Timeout, Traits) for supported sources
Display Names	Simplified formatting; correctness prioritized over decorative markers

Closes #49

[Copilot]

Pull Request Overview

This PR modernizes the Atc.Test library by upgrading from xUnit v2 to xUnit v3 and introduces a major version bump to 2.0. The upgrade enhances data attribute capabilities with async extensibility, implements deterministic frozen value handling with exact-type promotion, and improves safety around specimen reuse.

Key Changes

• xUnit v3 Migration: Complete migration of data attributes to use async ValueTask<IReadOnlyCollection<ITheoryDataRow>> extensibility model
• Enhanced Frozen Logic: Introduced two-phase frozen handling with positional reuse and exact-type promotion for member data attributes
• Infrastructure Modernization: Updated CI pipelines to .NET 9, comprehensive README restructuring, and improved developer experience

Reviewed Changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
version.json	Major version bump from 1.1 to 2.0 reflecting breaking changes
src/Atc.Test/Atc.Test.csproj	Updated dependencies to xUnit v3 and added .NET 9 target framework
src/Atc.Test/MemberAutoNSubstituteDataAttribute.cs	Complete rewrite to support async data generation with exact-type promotion
src/Atc.Test/ClassAutoNSubstituteDataAttribute.cs	Migrated to async model with positional-only frozen injection
src/Atc.Test/FrozenParameterInjector.cs	New shared helper centralizing frozen injection logic
test/Atc.Test.Tests/MemberAutoNSubstituteDataAttributeTests.cs	Added comprehensive tests for frozen reuse scenarios
README.md	Complete restructure with value proposition, compatibility notes, and usage examples
.github/workflows/	Updated CI workflows to use .NET 9

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[rickykaare]

Looks good 😊 Great job!