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

[JesperSchulz]

Summary

Adds six BC-platform-specific testing-domain knowledge articles under microsoft/knowledge/testing/, each as a trio (<slug>.md + <slug>.good.al + <slug>.bad.al) — 18 files total. Every article clears the README admission test: a modern LLM reviewing or generating BC test code would get the mechanic wrong without it. None drift into generic test-writing advice (AAA, naming, one-assert-per-test).

Originally drafted in community/knowledge/testing/; relocated to the microsoft/ layer per maintainer request (pure git mv, no content changes).

The six articles & admission-test rationale

P1 trio

• ui-calls-require-test-handlers — UI calls (Confirm, Message, Page.RunModal, StrMenu, Notification.Send, …) need a registered handler + [HandlerFunctions], or the test aborts with an unhandled-UI runtime error — not an assertion failure. LLMs routinely omit handlers.
• tablerelation-requires-prerequisite-records — Validate/Insert(true) enforces TableRelation; fixtures must be built parent-before-child or the test aborts on a relation error. LLMs assemble test data bottom-up.
• handlers-enqueue-never-assert — handlers run in a separate context; an assertion inside a handler can be swallowed, so the test passes while broken. Capture via LibraryVariableStorage.Enqueue, assert in the body, finish with AssertEmpty. Non-obvious platform behavior.

P2 trio

• handlerfunctions-attribute-must-match-ui-path — two-sided rule: a missing handler → unhandled UI; an unused listed handler → "handler function was not executed". Both are runtime failures. LLMs get the bidirectional contract wrong.
• asserterror-needs-expectederror-and-code — bare asserterror passes on any error; pair it with Assert.ExpectedError + Assert.ExpectedErrorCode to verify the specific failure. LLMs treat asserterror as sufficient.
• use-library-codeunits-for-test-fixtures — LibrarySales/LibraryInventory/… create valid records (number series, mandatory fields, relations); hand-rolled Init/Insert skips required setup and rots as the schema evolves. LLMs invent Init/Insert fixtures.

Articles mirror the format/frontmatter of the existing microsoft/knowledge/testing/transactionmodel-attribute-governs-test-transactions article (6-field frontmatter, domain: testing, bc-version: [all], technologies: [al], countries: [w1], application-area: [all]; code in sibling .al files only). Each .md is 24–26 lines (well under the 100-line cap).

Validator outputs

$ python .github/scripts/validate_frontmatter.py --root .
Validator: 0 error(s), 0 warning(s)        (exit 0)

$ pwsh .github/scripts/Test-KnowledgeIndex.ps1 -Root .
BCQuality index: 198 article(s). ...
Knowledge-index check PASSED: 198 articles, deterministic, full coverage, selection inputs intact.   (exit 0)

$ pwsh ./tools/Build-KnowledgeIndex.ps1
BCQuality index: 198 article(s). Index: .../knowledge-index.json
198        (exit 0)

Article count invariant

Baseline on main: 192 articles → after adding 6: 198 articles. Delta = +6. (The community→microsoft relocation is a move, so it leaves the count unchanged at 198.)

CODEOWNERS note

These files now live under microsoft/knowledge/testing/, which CODEOWNERS gates with the testing domain experts (@nikolakukrika @ventselartur @pchriste-microsoft-com). The PR therefore auto-requests them and is expected to sit BLOCKED on the 2-approval ruleset until they review. CODEOWNERS was not edited.

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Do not merge — open for maintainer review.