documentation: missing XML docs on core public APIs

[thomhurst]

Description

Several core public types lack XML documentation:

AbstractExecutableTest.cs (Lines 10-44)

public required string TestId { get; init; }  // No summary
public TestMetadata Metadata { get; init; } = null!;  // No summary
public required object?[] Arguments { get; init; }  // No summary

Other affected files

• TUnit.Core/Attributes/BaseTestAttribute.cs — many public properties without docs
• Multiple public methods/properties across TUnit.Assertions
• AbstractDynamicTest.cs — no docs

IntelliSense shows no helpful information for these types. Some types like AsyncEvent<T> have excellent documentation, showing the standard isn't applied consistently.

Suggested Fix

Add /// <summary> XML documentation blocks to all public members on core types.