Add assertions on JsonElement

[vbreuss]

Background and motivation

As discussed in #2205 we want support for System.Text.Json. I would suggest to keep it as similar as possible to the existing extension package FluentAssertions.Json.

API Proposal

Add a new class JsonElementAssertions which contains assertion methods on JsonElement?).
This class can then be extended with assertions similar to JTokenAssertions in FluentAssertions.Json.

In the first step, I would keep the public surface quite small and only add the following methods:

/// <summary>
/// Asserts that the number of items in the current <see cref="JsonElement" /> matches the supplied <paramref name="expected" /> amount.
/// </summary>
public AndConstraint<TAssertions> HaveCount(int expected, string because = "", params object[] becauseArgs);

/// <summary>
/// Asserts that the number of items in the current <see cref="JsonElement" /> does not match the supplied <paramref name="unexpected" /> amount.
/// </summary>
public AndConstraint<TAssertions> NotHaveCount(int unexpected, string because = "", params object[] becauseArgs);

/// <summary>
/// Asserts that the current <see cref="JsonElement" /> has a direct child element with the specified <paramref name="expected" /> name.
/// </summary>
public AndWhichConstraint<TAssertions, JsonElement> HaveElement(string expected, string because = "", params object[] becauseArgs)

/// <summary>
/// Asserts that the current <see cref="JsonElement" /> has no direct child element with the specified <paramref name="expected" /> name.
/// </summary>
public AndConstraint<TAssertions> NotHaveElement(string expected, string because = "", params object[] becauseArgs)

In order to use these assertions, I would add the following methods to the AssertionExtensions:

public static JsonElementAssertions Should(this JsonDocument actualValue);
public static JsonElementAssertions Should(this JsonElement? actualValue);

API Usage

var subject = JsonDocument.Parse("""[ "Hello", "World!" ]""");
subject.Should().HaveCount(2);

var subject = JsonDocument.Parse("""{ "id": 42 }""");
subject.Should().HaveElement("id");

When #2556 is also implemented, this could also be used as following:

string subject = """[ "Hello", "World!" ]""";
subject.Should().BeValidJson().Which.Should().HaveCount(2);

string subject = """{ "id": 42 }""";
subject.Should().BeValidJson().Which.Should().HaveElement("id");

Alternative Designs

No response

Risks

No response

Are you willing to help with a proof-of-concept (as PR in that or a separate repo) first and as pull-request later on?

Yes, please assign this issue to me.

[dennisdoomen]

Great. @jnyrup ?

[IT-VBFK]

What about the negative of HaveCount?

[vbreuss]

What about the negative of HaveCount?

That's a good question (the negative is also missing in JTokenAssertions), but it would probably be a good idea to add both (to be symmetrical).