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.md

@@ -15,7 +15,7 @@ application-area: [all]
 
 ## Best Practice
 
-Follow every `asserterror` with a verification of the error it expects: assert the action raises, then `Assert.ExpectedError` with the message — or a stable substring of it — and, where the code is known, `Assert.ExpectedErrorCode` (for example `'TestField'` for a mandatory-field check). Prefer matching on a code or an invariant fragment over the full localized sentence so the test survives caption changes without going blind to the wrong error.
+Follow every `asserterror` with a verification of the error it expects, and prefer the reusable `Library Assert` helpers over hardcoded literals. For a mandatory-field check, `Assert.ExpectedTestFieldError(FieldCaption, ExpectedValue)` encapsulates both the message and the `TestField` code, so the test survives caption or code changes and does not repeat that knowledge in every method. For other errors, pair `Assert.ExpectedError` with a stable substring — ideally a shared `Label`, not an inline sentence — and, where known, `Assert.ExpectedErrorCode`. When a needed check is missing from the shared library, extend `Library Assert` (or your own assert library) with a helper rather than hardcoding message text and codes across tests; matching on a code or an invariant fragment keeps the test from going blind to the wrong error when a caption is localized.
 
 See sample: `asserterror-needs-expectederror-and-code.good.al`.