Fix All().Satisfy() compile error with nullable value types

[Copilot]

All().Satisfy() fails to compile when the mapped property is a nullable value type (e.g., double?). The Satisfy lambda expects Assertion<TMapped>? as return type, but IsNotNull() on double? returns NotNullAssertion<double> (i.e., Assertion<double>), which is not assignable to Assertion<double?>.

// This fails to compile
await Assert.That(items)
    .All()
    .Satisfy(x => x.Price, c => c.IsNotNull()); // Price is double?

Approach

Uses a generic TAssertion type parameter with where TAssertion : IAssertion constraint on the Satisfy methods. This preserves type safety (the compiler still enforces that the lambda returns an assertion) while allowing type-transforming assertions like IsNotNull() on nullable value types.

Changes

• CollectionAssertions.cs: Added TAssertion type parameter with IAssertion constraint to CollectionAllSatisfyHelper.Satisfy overloads. The Satisfy methods wrap the typed lambda into Func<..., IAssertion?> when constructing the assertion classes. Internal storage in CollectionAllSatisfyAssertion and CollectionAllSatisfyMappedAssertion changed from Func<..., Assertion<T>?> to Func<..., IAssertion?>.
• NullAssertion.cs: NotNullAssertion.CheckAsync now checks metadata.Exception — required because the nullable-to-non-nullable Map throws InvalidOperationException on null, and the exception was being silently ignored since default(double) is never null.
• SatisfiesTests.cs: Added tests for All().Satisfy() with double? members (both pass and fail cases, direct and mapped).
• Public API snapshots: Updated all 4 .verified.txt files.

Fixes #4684

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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Review Summary:

This PR cleanly fixes the compilation error with nullable value types in All().Satisfy() by:

1. Widening the assertion return type from Assertion<T>? to IAssertion? - This is an elegant architectural improvement that provides the necessary flexibility while maintaining backward compatibility (since Assertion<T> implements IAssertion).

2. Adding exception handling in NotNullAssertion to properly detect null-related failures when mapping nullable-to-non-nullable types.

3. Comprehensive test coverage for both passing and failing scenarios with nullable value types.

4. Properly updated snapshot files following TUnit's testing conventions.

The changes are focused, well-tested, and maintain the project's quality standards.

[claude]

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Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

The PR correctly fixes the type compatibility issue with nullable value types in All().Satisfy() by:

1. Widening the assertion function signature from Assertion? to IAssertion? (consistent with existing patterns like MappedSatisfiesAssertion)
2. Adding proper exception handling in NotNullAssertion to catch null mapping failures
3. Updating API snapshots appropriately with .verified.txt files
4. Including comprehensive test coverage for the fix

The changes are well-structured and follow TUnit's development guidelines.

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