fix(historian): #691.4 review — uncapped Gateway events reads no longer pre-size to int.MaxValue

GatewayHistorianDataSource.ReadEventsAsync sized its result list to
maxEvents, which is int.MaxValue for NumValuesPerNode = 0: every
uncapped Gateway events read (forward on master, and the new walked
reverse one) threw 'Array dimensions exceeded' and counted as a backend
failure. Capacity is bounded at 1024. The events walk delegate moved to
ReverseEventsReader so it is tested over the real Gateway source
(forward uncapped, reverse uncapped, reverse capped).

docs/Historian.md

@@ -787,6 +787,9 @@ says is complete carries no continuation point. `NumValuesPerNode == 0` stays un
     oversized-cluster over-fetch keeps exactly the events at `T` (the gateway's end is inclusive) in the same order.
   - **"More exists"** is found by walking for one event past the page (the walk has no source truncation flag to hand on).
   - **Uncapped** reverse reads are one forward read of the window, sorted and reversed.
+  - ⚠️ **Also fixed (#691.4 review): uncapped events reads on Gateway.** `NumValuesPerNode = 0` reaches the source as
+    `int.MaxValue`, and `GatewayHistorianDataSource` pre-sized its result list to it ("Array dimensions exceeded") — every
+    uncapped Gateway events read, forward too, failed the node and counted as a backend failure. The capacity is now bounded.
   - A timestamp holding more than `MaxTieClusterOverfetch` events ⇒ **`BadTooManyOperations`** for that node.
   - ⚠️ **Gateway prerequisite:** the gateway's `RuntimeDb:EventReadMaxRows` must be `0` (its default, unlimited) or larger than
     the walk's biggest read (`2N + 18` rows, a discovery read 1,026, a tie-cluster read up to `2·MaxTieClusterOverfetch + 4`).