fix(historian): #675 review follow-ups — SupportsReverseEvents capability, no catch-as-unsupported

- IHistorianDataSource.SupportsReverseEvents (default false; SqlServer/QuestDB true; Gateway/Null declare false).
  The node manager checks it BEFORE calling the source: an end-only capped events read becomes reverse only on
  a supporting backend (Gateway, the default, keeps the pre-#675 forward [MinValue, EndTime) paging), and an
  explicit reverse read on a non-supporting backend is BadHistoryOperationUnsupported without a call.
- Removed the broad catch (NotSupportedException) in the events path: a NotSupportedException from inside a
  backend is now logged as a backend fault (Error), not an 'unsupported' Warning.
- Null declares the reverse overload itself; guard test: every production IHistorianDataSource declares both
  the reverse overload and the capability (positive-control scan of all four backends).
- Conformance: capability reported + oversized reverse tie cluster (forward over-fetch reversed == reverse
  read order; capped reverse read stopping inside the cluster flags truncation) on SqlServer + QuestDB.
- docs/Historian.md: end-only capped events read is backward on SqlServer/QuestDB, forward on Gateway.

docs/Historian.md

@@ -646,14 +646,20 @@ says is complete carries no continuation point. `NumValuesPerNode == 0` stays un
   forward) whatever the resume request's details say. Reverse tie clusters are over-fetched forward and sliced from the newest
   end, exactly as Raw does. Before #675 a capped reverse read silently returned the EARLIEST N, ascending, Good, with no
   continuation point.
-- **End-only events reads read backward (#675).** `StartTime` unset (`DateTime.MinValue`) with an `EndTime` and
-  `NumValuesPerNode > 0` pages backward from `EndTime` over `(MinValue, EndTime]` — the same rule as Raw. Start-only reads still
-  page forward to an open upper bound (`DateTime.MaxValue`, UTC). Without a cap an end-only read keeps its forward
-  `[MinValue, EndTime)` meaning.
-- **Backends.** The server calls `IHistorianDataSource.ReadEventsAsync(…, bool reverse, …)`. SqlServer and QuestDB serve reverse
-  natively (`… > lo AND … <= hi ORDER BY time DESC, id DESC`). **Gateway and Null refuse a reverse read** (the interface default
-  throws `NotSupportedException`) and the node answers `BadHistoryOperationUnsupported` — the gateway streams ascending only and
-  rejects `end < start`, and draining the whole window to keep its tail would be exactly the unbounded read the cap prevents.
+- **End-only events reads: backward on SqlServer/QuestDB, forward on Gateway (#675).** `StartTime` unset
+  (`DateTime.MinValue`) with an `EndTime` and `NumValuesPerNode > 0` pages backward from `EndTime` over `(MinValue, EndTime]` —
+  the same rule as Raw — **only on a backend that reports `SupportsReverseEvents`** (SqlServer, QuestDB). On Gateway (the
+  default backend) and Null it keeps its pre-#675 forward `[MinValue, EndTime)` meaning, paged oldest-first, rather than
+  becoming a refused reverse read. Start-only reads page forward to an open upper bound (`DateTime.MaxValue`, UTC) everywhere.
+  Without a cap an end-only read is forward `[MinValue, EndTime)` everywhere.
+- **Backends.** The server checks `IHistorianDataSource.SupportsReverseEvents` and then calls
+  `ReadEventsAsync(…, bool reverse, …)`. SqlServer and QuestDB serve reverse natively (`… > lo AND … <= hi ORDER BY time DESC,
+  id DESC`) and report `true`. **Gateway and Null report `false`**: an explicit reverse read (`StartTime > EndTime`) is answered
+  `BadHistoryOperationUnsupported` **without calling the backend** — the gateway streams ascending only and rejects `end < start`,
+  and draining the whole window to keep its tail would be exactly the unbounded read the cap prevents. Because the refusal is a
+  capability check, not a caught exception, a `NotSupportedException` thrown from inside a backend (a driver library) is logged
+  as a backend fault like any other. Every implementation (and any future wrapper) must declare both members itself — the
+  interface defaults (`false` / throw) would silently disable reverse reads; `HistorianDataSourceReverseEventsGuardTests` pins it.
 - **Gateway open upper bound.** The `DateTime.MaxValue` (UTC) bound of a start-only forward read is pinned offline through the
   published client's proto `Timestamp` conversion (captured by an in-process gRPC server) and the FILETIME encoding
   (`GatewayEventBoundsTests`). What a live AVEVA Historian does with a year-9999 bound is still unverified (needs the VPN).