Address review: merge handler articles, adopt enqueue-driven pattern

Respond to @nikolakukrika's review on #62:

- Merge ui-calls-require-test-handlers, handlerfunctions-attribute-must-match-ui-path
  and handlers-enqueue-never-assert into a single ui-handlers-in-tests article.
- Adopt the enqueue-from-test / dequeue-and-assert-in-handler pattern using
  Assert.ExpectedConfirm/ExpectedMessage (substring match), with Initialize()
  clearing LibraryVariableStorage and AssertEmpty() proving exact call counts.
- asserterror sample now uses Assert.ExpectedTestFieldError + FieldCaption instead
  of hardcoded message/code; article text points to the library helpers.
- Drop the tablerelation article and fold its test-relevant ordering point
  (relations checked on Validate/Insert(true); build parents first) into
  use-library-codeunits-for-test-fixtures.

Article count 198 -> 195.

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

microsoft/knowledge/testing/asserterror-needs-expectederror-and-code.good.al

@@ -13,9 +13,12 @@ codeunit 50408 "Test AssertError Good"
         // [WHEN] a mandatory field is blank
         asserterror Customer.TestField(Name);
 
-        // [THEN] verify the SPECIFIC failure — message and code — not just "any error"
-        Assert.ExpectedError('Name must have a value');
-        Assert.ExpectedErrorCode('TestField');
+        // [THEN] verify the SPECIFIC failure through a reusable Library helper
+        // instead of hardcoding the localized message and the 'TestField' code.
+        // ExpectedTestFieldError centralizes that knowledge, so the test keeps
+        // working when the caption or code changes; FieldCaption avoids pinning
+        // the field name as a literal.
+        Assert.ExpectedTestFieldError(Customer.FieldCaption(Name), '');
     end;
 
     var