Dependency-free WrapWorkerExecution retry seam on the base classes (#94)

[Chris-Wolfgang]

Closes #94.

Summary

Adds a dependency-free retry seam — protected virtual WrapWorkerExecution(...) — to ExtractorBase, LoaderBase, and TransformerBase, wrapped around every worker invocation (both the no-progress and with-progress paths). The default is a no-op, so existing behaviour is unchanged; an override runs the worker through a retry / resilience strategy.

protected virtual IAsyncEnumerable<TSource> WrapWorkerExecution(
    Func<CancellationToken, IAsyncEnumerable<TSource>> workerFactory, CancellationToken token)
    => workerFactory(token); // default: no wrapping

The override receives a re-invocable worker factory — calling it again produces a fresh worker run, which is what lets a strategy retry a transient failure.

Design decision — Option 1 (dep-free), per maintainer

The issue recommended Option A (inject Polly's ResiliencePipeline), but Abstractions is intentionally zero third-party runtime dependencies and roots 8 downstream packages, so taking a Polly dependency here would force it on the whole fleet. Chosen approach: ship only the seam; a ready-made Polly integration lands as a separate opt-in Wolfgang.Etl.Polly package — tracked in #332.

Scope

• Stream-level retry (whole worker re-run) per the issue; item-level deferred (composes with the Add OnItemError hook + RecordsSkipped (dead-letter / error capture) to base classes #84 OnItemError hook later). Counter-reset and source re-enumeration semantics are documented on each seam.

Verification

• 426 unit tests pass (13 new RetrySeamTests: default no-op, seam-invoked-once on both paths, factory re-invocability, genuine transient-failure recovery, null guard — across all 3 bases).
• Stryker 100.00 %, 0 survivors (full project).
• Builds clean across all 11 TFMs; PublicAPI.Unshipped updated (3 protected-virtual entries, RS0017-validated).

Notes for release-prep (not in this PR)

• Targets vNext-plus-one (the 0.20.0 line). The version bump 0.19.0 → 0.20.0 and PackageValidationBaselineVersion → 0.19.0 are deferred to 0.20.0 release-prep, gated on 0.19.0 publishing. The [Unreleased] section here still carries the Widen EtlPipelineProgress counters from int to long (overflow-safe record counts) #285/Mutation survivors (10) — kill these to raise the score #328 entries inherited from the vNext fork; those get reconciled when main (0.19.0) merges into vNext-plus-one.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.