fix: generate random URLs for webhook-like recipients

[jharley]

The nightly "smoke test" failed last evening for two reasons:

1. we had an API-side bug when deleting certain shapes of recipients (now fixed)
2. because of the above, there were recipients that were considered duplicates as their Webhook URLs matched (affects Webhook and MSTeams* recipients). In the name of moving toward being able to run these tests concurrently, this fixes that duplicate issue.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.80%. Comparing base (24c6e3f) to head (4dd18b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
+ Coverage   78.78%   78.80%   +0.01%     
==========================================
  Files          86       86              
  Lines        6124     6128       +4     
==========================================
+ Hits         4825     4829       +4     
  Misses        964      964              
  Partials      335      335              

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