Add negative test for FS3885 non-empty invalid array in attribute

Verify that non-empty arrays of user-defined types in custom attributes
produce the proper FS3885 diagnostic instead of an internal error.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

tests/FSharp.Compiler.ComponentTests/Language/AttributeCheckingTests.fs

@@ -190,6 +190,28 @@ type C() = class end
         |> compile
         |> shouldSucceed
 
+    // https://github.com/dotnet/fsharp/issues/12796
+    // Non-empty arrays of user-defined types in custom attributes should produce FS3885 diagnostic.
+    // Previously this caused FS0192 internal error in encodeCustomAttrElemType.
+    [<Fact>]
+    let ``Issue 12796 - Non-empty array of user-defined type in attribute gives proper error`` () =
+        FSharp
+            """
+module TestModule
+
+open System.ComponentModel
+
+[<AllowNullLiteral>]
+type A() = class end
+
+[<DefaultValue([| (null : A) |])>]
+type B() = class end
+            """
+        |> compile
+        |> shouldFail
+        |> withErrorCode 3885
+        |> withDiagnosticMessageMatches "not a valid custom attribute argument type"
+
     // https://github.com/dotnet/fsharp/issues/12796
     // Empty arrays of user-defined types in custom attributes should compile successfully.
     // The element type is substituted with System.Object since no elements need encoding.