test(infra) Multimorphic scraper-pipeline integration tests

[jkeeley2073]

Summary

Backfill of the PR #41 family-wide test-infra template for MultimorphicProductScraper. 5 scraper-pipeline integration tests covering the audit-flagged invariants the unit-test surface cannot reach.

• Discovery shape: WordPress sitemap walk — /wp-sitemap.xml -> wp-sitemap-posts-product-1.xml -> per-product HTML pages, filtered to /store/p3-game-kits/multimorphic-game-kits/{slug}/.
• Yield kind: Single — .Game only (no .Link).
• Source: SourceType.MultimorphicProductPage, discovery context "Multimorphic Game Kit".

Tests added

1. ScrapeAsync_HappyPath_YieldsGamesInSitemapOrderWithProvenance — sitemap index XML -> product sub-sitemap XML (with third-party kit + circuit board mixed in to pin the prefix filter) -> 3 product HTML pages. Asserts: yield count + slug order, full provenance (SourceType, DiscoveryContext, DiscoveryUrl, Source.ScrapedFrom, ScrapedAt, DiscoveredOn), gate-vs-wire URL byte-equality, exact wire-trace (5 requests: index + sub-sitemap + 3 products, no third-party / circuit board).
2. ScrapeAsync_PerPageFetchFailure_DoesNotAbortRun — middle product page returns 500; siblings still yield; politeness invariants hold under failure (the 500 is reported back).
3. ScrapeAsync_DiscoveryFailure_AbortsThisSourceOnly — sitemap index returns 500; scraper yields nothing AND does not throw (clean yield-break for orchestrator).
4. ScrapeAsync_PolitenessExceptionFromGate_PropagatesUp — ThrowOnAcquire injects PolitenessException; wire shows zero requests, gate shows zero reports.
5. ScrapeAsync_GateThrowsOnReport_BubblesUp — symmetric: ThrowOnReport propagates.

Test count: 430 -> 435.

Pre-push self-audit

• /local-review: deferred to the reviewer at merge time (per task instruction).
• 7-item mechanical checklist: all pass.
  1. No new options added (tests-only PR).
  2. Sibling-diff vs CGC template: structure matches; adapted for XML sitemap fixtures, single-yield, sitemap-walk wire trace.
  3. No bare catch { }.
  4. CLI / orchestrator wiring: unchanged (multimorphic alias already wired in PR feat(multimorphic) Multimorphic scraper — Phase 1.3.c #39).
  5. Tests assert behavior — happy-path pins yield order + provenance content + URL equality; failure tests pin the broken slug is excluded AND the 500 is reported; politeness tests pin zero downstream requests.
  6. Build: 0 warnings, 0 errors.
  7. Identity: Jim Keeley <94459922+jkeeley2073@users.noreply.github.com> (personal noreply).

Test plan

• dotnet build clean (0 warnings)
• New tests pass (5/5)
• Full suite passes (435/435)
• Reviewer to run /local-review at merge time

Hard constraints honored

• Touched only the new test file + a single CHANGELOG bullet.
• No production code changed.
• No scraper bugs found.