Add synchronous JsonSchema.FromJson methods

[lahma]

Summary

• Add synchronous JsonSchema.FromJson() overloads as alternatives to FromJsonAsync() for schemas that don't require external file or URL reference resolution
• Internal document references (#/definitions/...) are resolved synchronously
• External file/URL references throw NotSupportedException with a message directing users to use FromJsonAsync instead
• Adds sync UpdateSchemaReferences in JsonSchemaReferenceUtilities and sync ResolveReference in JsonReferenceResolver

Fixes #1727

New API

JsonSchema.FromJson(string data)
JsonSchema.FromJson(string data, string? documentPath)
JsonSchema.FromJson(string data, string? documentPath, 
    Func<JsonSchema, JsonReferenceResolver> referenceResolverFactory)

Test plan

• Basic schema parsing works synchronously
• Internal $ref references (#/definitions/...) resolve correctly
• Sync FromJson produces identical results to FromJsonAsync for self-contained schemas
• All existing tests continue to pass

🤖 Generated with Claude Code