fix: WR-18424 support the payload attribute being received as a serialized JSON object

[DustinK-retailsuccess]

The following test message was received and logged the following error message.

This adjusts our logic to align with the expected payload. ProPay indicates this difference from their docs was caused by the payload being encrypted on their side and they hadn't seen the output exercised yet to understand the change in output.

NOTE: This also adjusts what we store to just be the payload attribute of the detail received in the webhook. The other attributes on the detail seem irrelevant or redundant from the information we already track for a ProPay enabled store.

{"version":"0","id":"ad917211-80b0-12f4-e6c2-6ed93c2f8c0b","detail-type":"merchantware.credentials.created","source":"propay.webhook","account":"203538442868","time":"2026-06-05T21:05:00Z","region":"us-east-1","resources":[],"detail":{"notificationId":"e7982070-8c25-448f-a6c4-dc63d8e4eae0","eventType":"merchantware.credentials.created","eventDateTimeUTC":"06/05/2026 21:04:57","payload":"{\"accountNum\":1234564789,\"merchantId\":123456789,\"merchantName\":\"Kurt T2P Test\",\"tapToPay\":{\"terminalId\":10000036,\"activationCode\":\"some-redacted-code\",\"merchantSiteId\":\"1QH5PTEV\",\"merchantKey\":\"AUDKZ-4I0SS-SJBN2-UD6DW-JCCZW\"}}"}}

{
    "Timestamp": "2026-06-05T21:05:03.0940340+00:00",
    "Level": "Error",
    "MessageTemplate": "Unrecoverable error processing message. Dead-lettering.",
    "Exception": "System.InvalidOperationException: The requested operation requires an element of type 'Object', but the target element has type 'String'.\n   at System.Text.Json.ThrowHelper.ThrowJsonElementWrongTypeException(JsonTokenType expectedType, JsonTokenType actualType)\n   at System.Text.Json.JsonDocument.TryGetNamedPropertyValue(Int32 index, ReadOnlySpan`1 propertyName, JsonElement& value)\n   at Wayroo.Payments.ConfigurationRecorder.Lambda.Gateways.Propay.PropayConfigurationParser.TryReadDetail(String body, String& detailJson, Int64& accountNumber) in /home/vsts/work/1/s/Wayroo.Payments.ConfigurationRecorder.Lambda/Wayroo.Payments.ConfigurationRecorder.Lambda/Gateways/Propay/PropayConfigurationParser.cs:line 47\n   at Wayroo.Payments.ConfigurationRecorder.Lambda.Gateways.Propay.PropayConfigurationParser.Parse(String rawWebhookBody) in /home/vsts/work/1/s/Wayroo.Payments.ConfigurationRecorder.Lambda/Wayroo.Payments.ConfigurationRecorder.Lambda/Gateways/Propay/PropayConfigurationParser.cs:line 20\n   at Wayroo.Payments.ConfigurationRecorder.Lambda.PaymentConfigurationMessageHandler.Handle(SQSMessage message, CancellationToken cancellationToken) in /home/vsts/work/1/s/Wayroo.Payments.ConfigurationRecorder.Lambda/Wayroo.Payments.ConfigurationRecorder.Lambda/PaymentConfigurationMessageHandler.cs:line 39\n   at Wayroo.Payments.ConfigurationRecorder.Lambda.Function.FunctionHandler(SQSEvent input, ILambdaContext context) in /home/vsts/work/1/s/Wayroo.Payments.ConfigurationRecorder.Lambda/Wayroo.Payments.ConfigurationRecorder.Lambda/Function.cs:line 54",
    "Properties": {
        "SourceContext": "Wayroo.Payments.ConfigurationRecorder.Lambda.ProcessFailureHandler",
        "MessageId": "0d18262e-0223-4aea-be9b-a40adcf8b90a",
        "TraceId": "Root=1-6a2339fc-9aadf74b36463538912b22f7;Parent=734b0a00ef7eff47;Sampled=0;Lineage=1:008a1e6d:0",
        "FunctionName": "dev-WayrooPayments-ConfigurationRecorderLambda",
        "AwsRequestId": "64ca1f07-c0d5-5f12-82f6-7d9afcfae41d",
        "InvokedFunctionArn": "arn:aws:lambda:us-east-1:203538442868:function:dev-WayrooPayments-ConfigurationRecorderLambda",
        "Scope": [
            "0d18262e-0223-4aea-be9b-a40adcf8b90a",
            "0d18262e-0223-4aea-be9b-a40adcf8b90a"
        ]
    }
}