Add testing knowledge: UI handlers, table relations, asserterror, fixtures (P1+P2)

Six BC-specific testing-domain knowledge articles in community/knowledge/testing/, each with .good.al/.bad.al samples:

- ui-calls-require-test-handlers
- tablerelation-requires-prerequisite-records
- handlers-enqueue-never-assert
- handlerfunctions-attribute-must-match-ui-path
- asserterror-needs-expectederror-and-code
- use-library-codeunits-for-test-fixtures

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

community/knowledge/testing/ui-calls-require-test-handlers.bad.al

@@ -0,0 +1,26 @@
+codeunit 50401 "Test UI Handlers Bad"
+{
+    Subtype = Test;
+
+    // No [HandlerFunctions] and no ConfirmHandler: the Confirm below has
+    // nothing to intercept it, so this test fails at runtime with an
+    // "unhandled UI" error before any assertion is evaluated.
+    [Test]
+    procedure DeleteDocumentConfirmsAndProceeds()
+    var
+        Deleted: Boolean;
+    begin
+        Deleted := TryDeleteWithConfirm();
+        Assert.IsTrue(Deleted, 'Routine should proceed after confirmation.');
+    end;
+
+    local procedure TryDeleteWithConfirm(): Boolean
+    begin
+        if not Confirm('Delete this document?', false) then
+            exit(false);
+        exit(true);
+    end;
+
+    var
+        Assert: Codeunit "Library Assert";
+}