[RemoteJob] lazy snapshot_id calculation (#25548)

wait to call job_index.snapshot_id until its needed

## How I Tested These Changes

existing coverage

python_modules/dagster/dagster/_core/remote_representation/external.py

@@ -503,14 +503,13 @@ def __init__(
         if job_data_snap:
             self._active_preset_dict = {ap.name: ap for ap in job_data_snap.active_presets}
             self._name = job_data_snap.name
-            self._snapshot_id = self._job_index.job_snapshot_id
 
         elif job_ref_snap:
             self._active_preset_dict = {ap.name: ap for ap in job_ref_snap.active_presets}
             self._name = job_ref_snap.name
             if ref_to_data_fn is None:
                 check.failed("ref_to_data_fn must be passed when using deferred snapshots")
-            self._snapshot_id = job_ref_snap.snapshot_id
+
         else:
             check.failed("Expected either job data or ref, got neither")
 
@@ -631,6 +630,13 @@ def metadata(self) -> Mapping[str, MetadataValue]:
     def job_snapshot(self) -> JobSnap:
         return self._job_index.job_snapshot
 
+    @property
+    def _snapshot_id(self) -> str:
+        if self._job_ref_snap:
+            return self._job_ref_snap.snapshot_id
+
+        return self._job_index.job_snapshot_id
+
     @property
     def computed_job_snapshot_id(self) -> str:
         return self._snapshot_id