fix: webhook source transformation failed with error: cannot find module

[atzoum]

Description

Source type should be set to lowercase while constructing the webhook url. Introduced by #4007
Adding an integration test to guard against a future regression.

Linear Ticket

Fixes PIPE-522

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f95fa51) 71.99% compared to head (3d02fc3) 72.33%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4120      +/-   ##
==========================================
+ Coverage   71.99%   72.33%   +0.33%     
==========================================
  Files         377      383       +6     
  Lines       55223    55243      +20     
==========================================
+ Hits        39760    39962     +202     
+ Misses      13142    12951     -191     
- Partials     2321     2330       +9     

Files	Coverage Δ
gateway/webhook/webhookTransformer.go	72.04% <100.00%> (ø)
testhelper/backendconfigtest/source_builder.go	86.66% <100.00%> (+2.05%)	⬆️

... and 32 files with indirect coverage changes

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