fix(auditlog): capture request/response payloads on outbound API audit rows

The outbound ApiCall emitter hard-coded RequestSummary/ResponseSummary to null,
so audited API calls carried no inputs/outputs — contrary to the Audit Log
payload-capture spec. Thread the call arguments into the sync ApiCall emitter
and the cached immediate-completion path (CachedSubmit / ApiCallCached /
CachedResolve), and stamp the response body from ExternalCallResult.ResponseJson.
The writer's payload filter still applies the size cap + redaction downstream.

The S&F retry-loop cached rows are unchanged — request data is not threaded
through the store-and-forward buffer (same boundary as SourceScript).

tests/ScadaLink.SiteRuntime.Tests/Scripts/ExternalSystemCallAuditEmissionTests.cs

@@ -81,6 +81,29 @@ public class ExternalSystemCallAuditEmissionTests
         Assert.Equal(DateTimeKind.Utc, evt.OccurredAtUtc.Kind);
         Assert.NotEqual(Guid.Empty, evt.EventId);
         Assert.False(evt.PayloadTruncated);
+        // No call arguments → null request summary; the response body is captured.
+        Assert.Null(evt.RequestSummary);
+        Assert.Equal("{}", evt.ResponseSummary);
+    }
+
+    [Fact]
+    public async Task Call_CapturesRequestArgs_AndResponseBody_OnTheAuditRow()
+    {
+        var client = new Mock<IExternalSystemClient>();
+        client
+            .Setup(c => c.CallAsync("Weather", "GetCurrent", It.IsAny<IReadOnlyDictionary<string, object?>?>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new ExternalCallResult(true, "{\"tempC\":11.4}", null));
+        var writer = new CapturingAuditWriter();
+
+        var helper = CreateHelper(client.Object, writer);
+        var args = new Dictionary<string, object?> { ["city"] = "Dublin" };
+        await helper.Call("Weather", "GetCurrent", args);
+
+        var evt = Assert.Single(writer.Events);
+        // RequestSummary is the serialized argument dictionary; ResponseSummary
+        // is the verbatim response body. (Cap + redaction are the writer's job.)
+        Assert.Equal("{\"city\":\"Dublin\"}", evt.RequestSummary);
+        Assert.Equal("{\"tempC\":11.4}", evt.ResponseSummary);
     }
 
     [Fact]