Add httpPostForm() method to HttpClientSupport

[matrei]

This pull request introduces a new helper method, httpPostForm, to the HttpClientSupport trait, making it easier to send application/x-www-form-urlencoded requests in integration tests.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.4352%. Comparing base (7da662f) to head (5aff68c).

Files with missing lines	Patch %	Lines
...rails/testing/http/client/HttpClientSupport.groovy	66.6667%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #15735        +/-   ##
==================================================
+ Coverage     48.4322%   48.4352%   +0.0030%     
  Complexity      15265      15265                
==================================================
  Files            1875       1875                
  Lines           85852      85855         +3     
  Branches        14972      14972                
==================================================
+ Hits            41580      41584         +4     
+ Misses          37863      37862         -1     
  Partials         6409       6409                

Files with missing lines	Coverage Δ
...rails/testing/http/client/HttpClientSupport.groovy	80.0000% <66.6667%> (-20.0000%)	⬇️

... and 2 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 5aff68c
▶️ Tests: 41848 executed
⚪️ Checks: 43/43 completed

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