Added new string assert functions

[OsirisTerje]

No description provided.

[Copilot]

Pull request overview

This PR adds documentation for four new StringAssert methods that provide convenient assertions for checking null, empty, and whitespace conditions in strings. These methods are classic-style assertions that complement the existing StringAssert API.

Key changes:

• Added IsNullOrEmpty and IsNotNullNorEmpty method signatures for null/empty string checks
• Added IsNullOrWhiteSpace and IsNotNullNorWhiteSpace method signatures for null/whitespace string checks
• All new methods include overloads with optional message and format arguments

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[Copilot]

The naming convention IsNotNullNorEmpty is inconsistent with the existing pattern in StringAssert. All existing negative assertion methods follow the pattern DoesNot* or AreNotEqual* (e.g., DoesNotContain, DoesNotStartWith, DoesNotEndWith, AreNotEqualIgnoringCase). Consider renaming to IsNotNullOrEmpty to maintain consistency with the positive version IsNullOrEmpty, or better yet, follow the existing naming pattern and use something like DoesNotHaveNullOrEmpty if such a pattern fits.

Suggested change

	StringAssert.IsNotNullNorEmpty(string? actual);
	StringAssert.IsNotNullNorEmpty(string? actual,
	StringAssert.IsNotNullOrEmpty(string? actual);
	StringAssert.IsNotNullOrEmpty(string? actual,

[Copilot]

The naming convention IsNotNullNorWhiteSpace is inconsistent with the existing pattern in StringAssert. All existing negative assertion methods follow the pattern DoesNot* or AreNotEqual*. Additionally, the positive version uses "Or" (IsNullOrWhiteSpace) but the negative version uses "Nor", which is inconsistent. Consider renaming to IsNotNullOrWhiteSpace to maintain consistency with the positive version IsNullOrWhiteSpace.

Suggested change

	StringAssert.IsNotNullNorWhiteSpace(string? actual);
	StringAssert.IsNotNullNorWhiteSpace(string? actual,
	StringAssert.IsNotNullOrWhiteSpace(string? actual);
	StringAssert.IsNotNullOrWhiteSpace(string? actual,