Add BeValidJson extensions for strings

[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 the following methods to the StringAssertions (similar to StringAssertionsExtensions in FluentAssertions.Json):

  public AndWhichConstraint<TAssertions, JsonElement> BeValidJson(string because = "", params object[] becauseArgs);
  public AndWhichConstraint<TAssertions, JsonElement> BeValidJson(JsonDocumentOptions options, string because = "", params object[] becauseArgs);

These methods should assert that the string is a valid JSON document.

API Usage

string subject = """{ "id": 42, "admin": true }""";
subject.Should().BeValidJson();

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]

I like that too. What about you @jnyrup ?

[dennisdoomen]

@jnyrup ?

[vbreuss]

I updated the proposal to use JsonElement instead of JsonDocument, as latter is IDisposable and should not be returned directly.
See this comment for more information.