Improvements and small bug fix in List Webhooks extension methods

[ypcode]

Q	A
Bug fix?	yes
New feature?	no
New sample?	no

What's in this Pull Request?

• Bug fix : Ensure List Id is loaded when needed
• Pre-check of expiration date validity
• Add a valid delta when validity period is 6 months exactly
• better handling of exception in WebhooksUtility methods

[msftclas]

@ypcode,
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[jansenbe]

Thanks Yannick, good change which I'll process.

[ypcode]

You are welcome. It is my pleasure to contribute

[jansenbe]

I did have a peek at how we calculate the max expiration data and added some code snippet below. Can you adjust your PR to take today + 180 days as the max expiration. Internally we add a 90 minutes grace period to cater for time between the date being calculated and the webhook being added.

        internal const int expirationDateTimeMaxDays = 180;
        internal const int expirationDateTimeGraceMinutes = 90;

       DateTime utcNow = DateTime.UtcNow;
       // Maximum expiration enforced by this API includes a grace period
        DateTime maximumExpirationDateTime = utcNow
                .AddDays(SPWebhookSubscriptionInformation.expirationDateTimeMaxDays)
                .AddMinutes(SPWebhookSubscriptionInformation.expirationDateTimeGraceMinutes);

[ypcode]

Done. I also added 2 unit tests to verify the very last valid expiration and a barely invalid one

[jansenbe]

Thanks. I've dropped the expirationDateTimeGraceMinutes from your code as we introduced that grace time for server side only.

[ypcode]

Oh OK, I missed it was only internal. Thanks for the merge.