feat(batching): ProbeIndividuallyAfter(N) count-based batch member isolation (GH-3289 Phase 3c)

[jeremydmiller]

Phase 3c of the #3289 batch-processing plan: item 2 (ProbeIndividuallyAfter), the count-based sibling of item 3's IsolateBatchMembers (#3311). (Phases 0/1/2/3a/3b all merged.)

The feature

When you don't have a specific exception type to key on — the batch just fails and you can't tell transient from poison — configure a count-based probe directly on the batch:

opts.BatchMessagesOf<Order>(b =>
{
    b.BatchSize = 500;
    b.ProbeIndividuallyAfter(attempts: 3); // retry the whole batch 3×, THEN probe each member
});

It retries the whole batch N times (so a transient failure clears on retry), and only then re-runs each member as its own size-1 batch so only the genuinely bad one dead-letters.

Implementation

• ProbeIndividuallyContinuation gains a probeAfterAttempts threshold: it requeues the whole batch (DeferAsync) while Envelope.Attempts < threshold, then probes into singletons. The size-≤1 guard still dead-letters immediately (terminating case + probed singletons). IsolateBatchMembers keeps threshold 1 (probe on first failure) — behavior unchanged, verified by re-running its tests.
• BatchingOptions.ProbeIndividuallyAfter(N) stores the threshold; a new startup step applyBatchProbePolicies() (next to the existing warnOrAssertBatchHandlerConflicts) installs it as an every-attempt failure rule on the batch handler chain via batchChain.OnException<Exception>().ContinueWith(new ProbeIndividuallyContinuationSource(N)).

Test (DB-free)

batch_probe_individually_after.cs: the whole 3-item batch is retried exactly 3 times (asserted), then the probe isolates the poison item — the two healthy members succeed as singletons and only bad is dead-lettered (captured via IDeadLetterInterceptor). Item 3 + 3a isolation tests still green; dotnet build wolverine.slnx -c Release clean.

Remaining plan follow-up

Only the CoalesceBy same-key poisoning refinement for ApplyItemException (flagged in #3302) remains — a small final PR.

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