NIFI-15902: Ensure subject is included in merged clustered response f…

[mcgilman]

…or connector verification.

Summary

NIFI-15902

When a connector configuration step is verified on a clustered NiFi, the
subject field on each ConfigVerificationResultDTO is dropped during
cluster aggregation. As a result, clients receive verification results
with a populated step name, outcome, and explanation but a null
subject, even though every node produced a subject for the failing
property.

The cause is in ConfigVerificationResultMerger.computeAggregateResults(),
which constructs a brand new aggregate DTO and copies only
verificationStepName, outcome, and explanation from the selected
node result. The subject is silently lost.

This change copies the subject from the selected node result onto the
aggregate DTO so it is returned to the client unchanged. The "selected"
result is already chosen as the worst-outcome result (FAILED > SKIPPED >
SUCCESSFUL), which means the surfaced subject corresponds to the
verification result the user actually needs to act on.

The same ConfigVerificationResultMerger is also used by
VerifyConfigEndpointMerger (processors, controller services,
reporting tasks, parameter providers, flow analysis rules), so any of
those endpoints whose upstream DTOs include a subject will now also
preserve it through cluster aggregation. Today their per-DAO DTO
converters do not set subject, so there is no behavior change for
those endpoints; only the connector verification path observes a
visible difference.

Behavior

• Single-node responses: explanation, outcome, step name, and subject
  are all preserved (unchanged).
• Multi-node clusters where all nodes agree: the agreed subject is
  returned (unchanged step name/outcome/explanation handling).
• Multi-node clusters where nodes disagree: the subject from the
  worst-outcome (selected) result is returned, consistent with how
  the explanation prefix is already chosen.

Tests

ConfigVerificationResultMergerTest adds three cases covering the new
behavior:

• testSingleNodeSubjectPreserved - single-node failed result keeps
  its subject through the merge.
• testMultiNodeAgreeSubjectPreserved - two nodes agreeing on a
  successful result keep the shared subject.
• testMultiNodeDisagreeSubjectFromSelectedResult - when one node
  succeeds and another fails, the merged result carries the failing
  node's subject, matching the worst-outcome selection that already
  governs the merged outcome and explanation.