fix: use FirstOrDefault instead of foreach+if in FindDocumentByBaseUri

Addresses CodeQL feedback. Also resolves merge conflict with #2918
(RegisterSchemaIdentifiers + RegisterAnchors called together).

Author: aqeelat

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -662,12 +662,10 @@ private void RegisterAnchor(OpenApiDocument document, string anchorName, IOpenAp
         internal OpenApiDocument? FindDocumentByBaseUri(string documentUri)
         {
             if (!Uri.TryCreate(documentUri, UriKind.Absolute, out var uri)) return null;
-            if (_dynamicAnchorRegistryByDocument is not null)
-                foreach (var doc in _dynamicAnchorRegistryByDocument.Keys)
-                    if (doc.BaseUri == uri) return doc;
-            if (_anchorRegistryByDocument is not null)
-                foreach (var doc in _anchorRegistryByDocument.Keys)
-                    if (doc.BaseUri == uri) return doc;
+            if (_dynamicAnchorRegistryByDocument?.Keys.FirstOrDefault(doc => doc.BaseUri == uri) is { } dynMatch)
+                return dynMatch;
+            if (_anchorRegistryByDocument?.Keys.FirstOrDefault(doc => doc.BaseUri == uri) is { } anchorMatch)
+                return anchorMatch;
             return null;
         }