FINERACT-2621: Add CI-only diagnostics for missing external events in e2e

[avivijay19]

Description

Extracted from #6069, where review asked for these e2e changes to be raised in a separate PR since they are not part of the Spring Boot upgrade itself.

• When an event assertion times out, the step now queries the internal external events endpoint and logs whether the event was never persisted to m_external_event or was persisted but never delivered to the broker. A timeout alone cannot tell these two stages apart, which makes intermittent CI failures hard to attribute. Controlled by EVENT_FAILURE_DIAGNOSTICS_ENABLED, off by default and set only in the e2e workflow, and it only runs after an assertion has already failed, so passing runs are unaffected.
• loanAccountDataV1Check consumes the latest received event of the asserted type, which can be a stale predecessor when earlier steps raised events of the same type for the same loan. The verification is now retried so each attempt consumes the next event as it arrives.

Checklist

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