Support relative URI resolution in $dynamicRef

Problem

$dynamicRef values with relative URIs (e.g., ./external.yaml#node) are not resolved. FindDocumentByBaseUri compares the raw string against document base URIs, so a relative reference never matches an absolute document URI.

Example:

# Document A (baseUri: https://example.com/main.yaml)
Tree:
  properties:
    next:
      $dynamicRef: './external.yaml#node'

# Document B (baseUri: https://example.com/external.yaml)
ExternalNode:
  $dynamicAnchor: node
  type: object

$dynamicRef: './external.yaml#node' should resolve to https://example.com/external.yaml (Document B), but FindDocumentByBaseUri('./external.yaml') cannot match https://example.com/external.yaml.

Proposed fix

Pass the host document's BaseUri to FindDocumentByBaseUri and resolve the relative reference via new Uri(baseUri, documentUri):

internal OpenApiDocument? FindDocumentByBaseUri(string documentUri, Uri? baseUri)
{
    var resolved = baseUri is not null 
        ? new Uri(baseUri, documentUri).ToString() 
        : documentUri;
    return _dynamicAnchorRegistryByDocument.Keys
        .Concat(_anchorRegistryByDocument.Keys)
        .Distinct()
        .FirstOrDefault(doc => doc.BaseUri?.ToString()
            .Equals(resolved, StringComparison.OrdinalIgnoreCase) == true);
}