test: add CurrentItemCount per-run reset tests to idempotency bases (#15)

[Chris-Wolfgang]

Adds the 3 previously-deferred CurrentItemCount-reset tests to the opt-in idempotency contract bases (the companions deferred in #15 pending ETL-Abstractions#246). #246 is resolved and live in Wolfgang.Etl.Abstractions 0.14.0 (already referenced on vNext): CurrentItemCount/CurrentSkippedItemCount now reset at the start of each run.

Tests added (one [Fact] per base)

• IdempotentExtractorContractTests.ExtractAsync_when_called_twice_CurrentItemCount_resets_Async
• IdempotentLoaderContractTests.LoadAsync_when_called_twice_CurrentItemCount_resets_Async
• IdempotentTransformerContractTests.TransformAsync_when_called_twice_CurrentItemCount_resets_Async

Each runs the SUT twice on the same instance, then asserts CurrentItemCount equals a single run's item count (not 2x), proving the per-run reset contract introduced in Abstractions 0.14.0 (ETL-Abstractions#246). The stale // NOTE: deferred pending #246 comment in each base was replaced with a note that the reset is now verified.

How CurrentItemCount is accessed

CurrentItemCount is declared only on the generic Abstractions base classes (ExtractorBase<,>, LoaderBase<,>, TransformerBase<,,>); there is no non-generic base that exposes it.

• Extractor — the base already had a TProgress type parameter, so I tightened its constraint from the interfaces to where TSut : ExtractorBase<TItem, TProgress>, ... (matching the existing ExtractorBaseContractTests style) and read sut.CurrentItemCount directly. No new type parameter, no break to the existing consumer subclass.
• Loader / Transformer — these bases have no TProgress type parameter. Adding one (to constrain to LoaderBase<TItem, TProgress> / TransformerBase<TItem, TItem, TProgress>) would break the existing two-arg consumer subclasses, and constraining to a hardcoded Report progress (LoaderBase<TItem, Report>) triggered a spurious RS0016/RS0017 pair on the unrelated, already-shipped-on-vNext TryGetLoadedItems API entry. Per the CLAUDE.md guidance to prefer a small protected hook when tightening would break subclasses, I added a protected virtual int GetCurrentItemCount(TSut sut) to each base. Its default reads CurrentItemCount off LoaderBase<TItem, Report> / TransformerBase<TItem, TItem, Report> (the ETL convention) and is overridable for non-Report components. The existing consumer subclasses use Report, so no consumer code changed.

PublicAPI.Unshipped.txt entries added

Wolfgang.Etl.TestKit.Xunit.IdempotentExtractorContractTests<TSut, TItem, TProgress>.ExtractAsync_when_called_twice_CurrentItemCount_resets_Async() -> System.Threading.Tasks.Task!
Wolfgang.Etl.TestKit.Xunit.IdempotentLoaderContractTests<TSut, TItem>.LoadAsync_when_called_twice_CurrentItemCount_resets_Async() -> System.Threading.Tasks.Task!
virtual Wolfgang.Etl.TestKit.Xunit.IdempotentLoaderContractTests<TSut, TItem>.GetCurrentItemCount(TSut sut) -> int
Wolfgang.Etl.TestKit.Xunit.IdempotentTransformerContractTests<TSut, TItem>.TransformAsync_when_called_twice_CurrentItemCount_resets_Async() -> System.Threading.Tasks.Task!
virtual Wolfgang.Etl.TestKit.Xunit.IdempotentTransformerContractTests<TSut, TItem>.GetCurrentItemCount(TSut sut) -> int

Build & test

• dotnet build src/Wolfgang.Etl.TestKit.Xunit -c Release — Build succeeded, 0 warnings / 0 errors across all TFMs.
• dotnet test tests/Wolfgang.Etl.TestKit.Xunit.Tests.Unit -c Release -f net8.0 — Passed! 236/236. The 3 new reset tests pass, confirming 0.14.0 resets the count per run.

Tests only — no production code or test doubles changed.

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