test: raise Wolfgang.Etl.TestKit coverage to ≥90%

[Chris-Wolfgang]

Why

The vNext → main release PR (#187) fails CI's per-assembly 90% coverage gate: the Wolfgang.Etl.TestKit assembly sat at 80.3%. This PR adds test-only coverage to clear the gate and unblock #187.

Result

Assembly Wolfgang.Etl.TestKit: 80.3% → 99.13% line coverage.

Per-class (before → after):

Class	Before	After
FaultyExtractor	72.8%	100%
FaultyTransformer	69.2%	100%
FaultyLoader	75%	100%
TestTransformer	83.3%	94.1%
TestLoader	86.8%	95.8%
TestExtractor	89.7%	98.7%

What was covered

The previously-uncovered paths, all shared by the six doubles:

1. Progress + injected-timer path — CreateProgressTimer wiring (both the _progressTimer is null → base and the !_progressTimerWired → subscribe branches), CreateProgressReport, and the Elapsed handler firing a progress report. Added on all three Faulty* doubles and the Test* doubles, plus the TestExtractor factory/indexed factory-with-timer constructor argument-validation branches.
2. Dispose(bool) unsubscribe branch — covered on all six doubles.
3. SkipItemCount / MaximumItemCount enforcement on the Faulty* doubles (skip-continue and max-break branches).

All new tests mirror the existing patterns (the *WithTimer nested subclass exposing the protected timer ctor, ManualProgressTimer + SynchronousProgress<Report> from Wolfgang.Etl.TestKit.Xunit). Test code only — no src/ production file was modified.

Verification (local)

• dotnet build -c Release: 0 warnings, 0 errors (warnings-as-errors) across all test TFMs.
• Full multi-TFM run: 153/153 passed on net6.0/7.0/8.0/9.0/10.0 and net462/472/48/481.
• Coverage (--collect:"XPlat Code Coverage", net8.0): assembly line-rate 0.9913; every double class ≥ 0.94.

🤖 Generated with Claude Code