Fix #4481: FetchForWriting without appending events shouldn't block unrelated inserts (master)

[jeremydmiller]

Summary

Closes #4481. Master companion to #4489 (the 8.0 PR).

When an inline aggregate's stream was fetched via FetchForWriting<TAggregate> and no events were subsequently appended, an unrelated document insert on the same session was failing with JasperFx.ConcurrencyException on SaveChangesAsync — the empty stream still flowed through the inline projection's apply step and queued a snapshot-update operation against the unchanged aggregate version, tripping the optimistic-concurrency check.

Symptom reproduced only when the projection overrode Evolve directly; the conventional Apply/Create shape wasn't affected.

Root cause

Upstream in JasperFxAggregationProjectionBase.AppliesTo (jasperfx repo, unchanged across the 1.x → 2.x rename):

public bool AppliesTo(IEnumerable<Type> eventTypes)
{
    // Have to do this because you don't know if any events catch
    if (AllEventTypes.Length == 0) return true;
    return eventTypes.Intersect(AllEventTypes).Any() || ...;
}

For Apply/Create projections AllEventTypes is populated from the discovered handler methods, so an empty stream's empty eventTypes collection cleanly evaluates to false and the stream is screened out by the intersection check in JasperFxSingleStreamProjectionBase.ApplyAsync. For Evolve-based projections AllEventTypes is empty, the Length == 0 early-out fires return true, and the empty stream slips through — the projection then writes a snapshot with the old expected version and a concurrency check fires when any other operation commits.

Fix (cherry-picked from #4489, with master-side reconciliation)

Same approach as the 8.0 PR — filter empty streams at the Marten-side appender entry points. Master diverges from 8.0 on the IInlineProjection.ApplyAsync contract:

• 8.0 takes IReadOnlyList<StreamAction>. The 8.0 fix materializes a List<> via .ToList().
• 9.0 (JasperFx.Events 2.x, after Runtime perf: widen IInlineProjection.ApplyAsync streams parameter to IEnumerable<StreamAction> #4306) takes IEnumerable<StreamAction>. The cherry-pick reconciles to use a lazy Where(x => x.Events.Any()) instead of materializing — preserves the no-allocation property that Runtime perf: widen IInlineProjection.ApplyAsync streams parameter to IEnumerable<StreamAction> #4306 introduced.

RichEventAppender.ProcessEventsAsync and QuickEventAppender.ProcessEventsAsync both updated.

Test plan

• fetch_for_writing_without_appending_does_not_block_unrelated_inserts test cherry-picked from the 8.0 PR; passes on master with the fix
• All 212 FullyQualifiedName~FetchForWriting tests pass on net10.0 with the fix
• No regressions vs. master baseline

Companion PR on 8.0: #4489.

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