Add IsAssignableFrom and IsNotAssignableFrom type assertions

[Copilot]

Adds IsAssignableFrom<T>() / IsNotAssignableFrom<T>() as counterparts to the existing IsAssignableTo<T>() / IsNotAssignableTo<T>().

• New assertion classes: IsAssignableFromAssertion<TTarget, TValue> and IsNotAssignableFromAssertion<TTarget, TValue> in TypeOfAssertion.cs, using actualType.IsAssignableFrom(typeof(TTarget))
• Interface: Added both methods to IAssertionSource<TValue>
• Implementations: All 16 assertion source classes (ValueAssertion, DelegateAssertion, AsyncDelegateAssertion, TaskAssertion, CollectionAssertionBase, MemoryAssertionBase, etc.) plus explicit interface implementations and NotSupportedException stubs in CountWrapper/LengthWrapper
• Public API snapshots: Updated all 4 verified snapshot files

// IsAssignableTo: value's type can be assigned to T (value is subtype of T)
await Assert.That(derived).IsAssignableTo<BaseClass>();

// IsAssignableFrom: value's type can accept T (value is supertype of T)
await Assert.That(baseInstance).IsAssignableFrom<DerivedClass>();

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Feature]: IsAssignableFrom needs to be completed with it's counterpart IsAssignableTo</issue_title>
<issue_description>### Problem Statement

This is a simple one:

TUnit supports IsAssignableFom()

It would be nice to also support its counterpart IsAssignableTo()

Proposed Solution

simply call IsAssignableFrom reversing the parameters

Alternatives Considered

No response

Feature Category

Test Discovery / Attributes

How important is this feature to you?

None

Additional Context

No response

Contribution

• I'm willing to submit a pull request for this feature</issue_description>

<agent_instructions>Make sure to update public API snapshots</agent_instructions>

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

• Fixes [Feature]: IsAssignableFrom needs to be completed with it's counterpart IsAssignableTo #4824

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.