fix(db): classify transient vs permanent SQL errors in Database.CachedWrite (#7)

CachedWrite buffered ALL write failures and retried forever, never returning a
synchronous failure to the script — permanent SQL errors (constraint/syntax/
permission) were treated as transient. Mirror the External-System API path:
attempt immediately, return Failed synchronously on permanent SQL errors (no
buffering), buffer only transient errors; the S&F retry path parks permanent
failures instead of retrying forever. New SqlErrorClassifier + PermanentDatabaseException.

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Services/IDatabaseGateway.cs

@@ -56,8 +56,17 @@ public interface IDatabaseGateway
     /// <param name="parameters">Optional SQL parameters for the statement.</param>
     /// <param name="originInstanceName">Optional name of the instance that originated the write.</param>
     /// <param name="cancellationToken">Cancellation token for the buffering operation.</param>
-    /// <returns>A task that represents the asynchronous operation.</returns>
-    Task CachedWriteAsync(
+    /// <returns>
+    /// M2.3 (#7): an <see cref="ExternalCallResult"/> mirroring the External-System
+    /// API path (<c>IExternalSystemClient.CachedCallAsync</c>). The write is
+    /// attempted immediately:
+    /// <list type="bullet">
+    ///   <item>immediate success → <c>Success=true, WasBuffered=false</c> (not buffered);</item>
+    ///   <item>permanent SQL error (constraint / syntax / permission) → <c>Success=false, WasBuffered=false</c> with an error message, returned synchronously and NOT buffered;</item>
+    ///   <item>transient SQL error (connection / timeout / deadlock / throttle) → buffered to store-and-forward, <c>Success=true, WasBuffered=true</c>.</item>
+    /// </list>
+    /// </returns>
+    Task<ExternalCallResult> CachedWriteAsync(
         string connectionName,
         string sql,
         IReadOnlyDictionary<string, object?>? parameters = null,