test: mirror V31 dynamic ref tests to V32 and fix workspace fallback

Adds 17 missing V32 test mirrors for full parity with V31 coverage.
Replaces dead '?? new OpenApiWorkspace()' fallback with null-forgiving
'!' in both V31 and V32 cross-document tests.

Author: aqeelat

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDynamicRefTests.cs

@@ -635,7 +635,7 @@ public async Task AbsoluteDynamicRefResolvesAcrossDocuments()
         docA.BaseUri = new("https://example.com/main");
         docB.BaseUri = new("https://example.com/external");
 
-        var workspace = docA.Workspace ?? new OpenApiWorkspace();
+        var workspace = docA.Workspace!;
         workspace.RegisterComponents(docA);
         workspace.RegisterComponents(docB);
 
@@ -1494,7 +1494,7 @@ public async Task ExternalDynamicRefResolvesToPlainAnchorInExternalDocument()
         docA.BaseUri = new("https://example.com/main");
         docB.BaseUri = new("https://example.com/external");
 
-        var workspace = docA.Workspace ?? new OpenApiWorkspace();
+        var workspace = docA.Workspace!;
         workspace.RegisterComponents(docA);
         workspace.RegisterComponents(docB);
 
@@ -1554,7 +1554,7 @@ public async Task ExternalDynamicRefWithRelativeUriResolvesAcrossDocuments()
         docA.BaseUri = new("https://example.com/main.yaml");
         docB.BaseUri = new("https://example.com/external.yaml");
 
-        var workspace = docA.Workspace ?? new OpenApiWorkspace();
+        var workspace = docA.Workspace!;
         workspace.RegisterComponents(docA);
         workspace.RegisterComponents(docB);