[GOBBLIN-2269] Surface snapshot/partition copy metrics in observability (#4206)

* [GOBBLIN-XXXX] Surface snapshot/partition copy metrics in observability

Add per-dataset snapshotsCommitted and partitionsCommitted counts to both the
GaaSJobObservabilityEvent DatasetMetric and the job-status JobStatistics REST
surface, so file/blob movement jobs (e.g. DDM Iceberg snapshot replication) can
report table snapshots and partitions committed.

DatasetMetric (GaaSJobObservabilityEvent.avsc) gains snapshotsCommitted and
partitionsCommitted (long, default -1 = unsupported/unknown). DatasetTaskSummary
carries the two counts as non-@NonNull fields so the existing 5-arg
RequiredArgsConstructor used by AbstractJobLauncher is preserved; they populate
via JSON deserialization for events that supply them, and toDatasetMetric maps
them through.

JobStatistics (executionStatistics) gains optional bytesWritten, recordsWritten,
filesCommitted, snapshotsCommitted and partitionsCommitted. JobStatusRetriever
reads them from the flat ddm.* copy-metric keys the JobSummary event persists
into the job state, and FlowExecutionResource sets each only when reported, so
jobs that do not emit them are unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Drop the unrelated flow-level 'issues' field from the flowexecutions snapshot

FlowExecution.pdl already declares 'issues' on master; this branch never touched
that file. The snapshot json was simply stale, so regenerating it picked the field
up as an incidental diff. Removing it keeps the PR scoped to the copy metrics.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix findbugs UWF_NULL_FIELD on the new DatasetTaskSummary fields

The 'Run static checks' job failed on :gobblin-runtime:findbugsMain with two
UWF_NULL_FIELD violations: snapshotsCommitted and partitionsCommitted were
declared with an explicit '= null' initializer and no in-repo writer, so
FindBugs correctly concluded every read returns null. They are normally
populated reflectively during JSON deserialization, which FindBugs cannot see.

Add @AllArgsConstructor to give native producers a real write path and drop the
redundant '= null' initializers (fields already default to null). The 5-arg
@RequiredArgsConstructor is untouched, so existing call sites still compile.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>