Calling OpenApiDocument.Parse without the `format` parameter should not fail

[darrelmiller]

Describe the bug
A clear and concise description of what the bug is.

Code To Reproduce

 OpenApiReaderRegistry.RegisterReader("yaml",new OpenApiYamlReader());

 var stringOpenApiDoc = """
openapi: 3.1.0
info:
 title: Sample API
 version: 1.0.0
paths: {}
""";

 var readResult = OpenApiDocument.Parse(stringOpenApiDoc);

Expected behavior
Document should parse without error.

Screenshots/Code Snippets

Error message returned

OpenAPI document is invalid:
Please provide the correct format, 'o' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.