fix(test): lock AllItemsReceived state in batch_processing tests

[jeremydmiller]

Summary

• MartenTests.batch_processing.end_to_end_with_tenancy has been failing on every recent Marten CI run with an AggregateException of NREs out of AllItemsReceived.IsCompleted. Root cause: in the multi-tenant variant the "blue" and "green" tenant batches complete in parallel and both call Record(EnvelopeRecord) from the tracking infrastructure on whichever thread the handler finishes on. The helper fed the records into a plain List<BatchItem> via AddRange, which is not thread-safe — concurrent callers race on the internal array's resize and writes, leaving null slots that surface as NRE when the polling thread reads r.Id in the IsCompleted predicate.
• Locks both Record (writer) and IsCompleted (reader) so the polling thread sees a consistent snapshot.
• No production code is touched — the lock lives entirely in the test helper class.

Repro / verification

• Without fix: 1/10 fail locally on a fast box; deterministically failing in CI on slower runners (same NRE on every recent Marten run that's been retried twice and still red — see e.g. runs 25434414670, 25391326016, 25380002472, 25373294046, 25492035820).
• With fix: 15/15 pass locally for end_to_end_with_tenancy; sibling end_to_end_with_durable (which uses the same helper) also passes.

Test plan

• dotnet test --filter FullyQualifiedName=MartenTests.batch_processing.end_to_end_with_tenancy — 15/15 passed
• dotnet test --filter FullyQualifiedName~MartenTests.batch_processing — 2/2 passed
• CI Marten run on this PR clears

Note

This PR is also serving to put the Marten CI under load while you watch for any timeout regression.

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