feat(inbound-api): method-script exception detail on the inbound audit row, never on the HTTP body (D3)

src/ZB.MOM.WW.ScadaBridge.InboundAPI/Middleware/AuditWriteMiddleware.cs

@@ -79,6 +79,16 @@ public sealed class AuditWriteMiddleware
     /// </summary>
     public const string AuditActorItemKey = "ZB.MOM.WW.ScadaBridge.InboundAPI.AuditActor";
 
+    /// <summary>
+    /// <see cref="HttpContext.Items"/> key used by the endpoint handler to publish the
+    /// method script's exception detail when the script threw (Flow Debugger step logs,
+    /// design 2026-09-09 D3). The executor deliberately sanitises the caller-facing
+    /// message, so this slot is the only place the real cause survives; it is consumed
+    /// once here, onto the inbound audit row's <c>ErrorDetail</c>, and never written to
+    /// the HTTP response. An exception that ESCAPED the pipeline takes precedence.
+    /// </summary>
+    public const string ErrorDetailItemKey = "ScadaBridge.Audit.ErrorDetail";
+
     /// <summary>
     /// ParentExecutionId: <see cref="HttpContext.Items"/> key under
     /// which this middleware stashes the inbound request's per-request
@@ -318,6 +328,17 @@ public sealed class AuditWriteMiddleware
                 headerDict[header.Key] = value;
             }
 
+            // Flow Debugger step logs (design 2026-09-09 D3): an exception that ESCAPED the
+            // pipeline is the authoritative failure, so it wins; otherwise the endpoint may
+            // have stashed the method script's exception (which the executor keeps out of the
+            // HTTP body). Absent both, the row carries no detail.
+            var errorDetail = thrown?.ToString();
+            if (errorDetail is null
+                && ctx.Items.TryGetValue(ErrorDetailItemKey, out var stashedErrorDetail))
+            {
+                errorDetail = stashedErrorDetail as string;
+            }
+
             var extra = JsonSerializer.Serialize(new
             {
                 remoteIp = ctx.Connection.RemoteIpAddress?.ToString(),
@@ -347,6 +368,7 @@ public sealed class AuditWriteMiddleware
                 httpStatus: statusCode,
                 durationMs: (int)Math.Min(durationMs, int.MaxValue),
                 errorMessage: thrown?.Message,
+                errorDetail: errorDetail,
                 requestSummary: requestBody,
                 responseSummary: responseBody,
                 payloadTruncated: payloadTruncated,