FINERACT-2900: Let the send-events job drain a backlog within one run

[jenny1371]

Description

JIRA: https://issues.apache.org/jira/browse/FINERACT-2900

SendAsynchronousEventsTasklet reads and sends exactly one batch per run and then returns FINISHED. With the defaults (cron every minute, external-event-batch-size = 1000) the external events job therefore sends at most 1000 events per minute, even though a run only takes 0.12-0.28 s.

This adds fineract.events.external.max-batches-per-run (env FINERACT_EXTERNAL_EVENTS_MAX_BATCHES_PER_RUN, default 1, so the current behavior is unchanged). A run reads up to that many batches. Each batch is sent and marked as sent before the next one is read, and the run stops as soon as a batch is not full (backlog drained). If a send fails, the failed batch stays TO_BE_SENT and the run stops, exactly as today.

Measured (custom image built from this branch, PostgreSQL + Kafka, backlog of 12,000 TO_BE_SENT events, batch size 1000, default cron)

Setting	Result
max-batches-per-run=1 (control, today's behavior)	exactly 1000 events per run, runs 60 s apart, so 12,000 events take 12 minutes
max-batches-per-run=20	all 12,000 events sent in one run, about 4 s of job time

Per 1000-event batch (DEBUG timings): read 10-15 ms, message creation 90-300 ms, Kafka send ~25 ms, mark as sent 10-20 ms.

Tests

SendAsynchronousEventsTaskletTest: default reads one batch; exactly N batches when the backlog is larger; stops at a partial batch; a failure on the 2nd batch marks only the first batch as sent. ./gradlew :fineract-provider:spotlessCheck and the test class pass on JDK 25.

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