Add unit tests for captcha classes

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Tests added?	✔️
Breaks BC?	❌
Fixed issues

What does this PR do?

Adds unit tests for CaptchaAction, CaptchaAsset, and CaptchaValidator.

Coverage

File	Tests	Lines	MSI
framework/captcha/CaptchaAction.php	37	98/125 (78%)	42% (101/240)
framework/captcha/CaptchaAsset.php	2	0/0 (100%)	n/a
framework/captcha/CaptchaValidator.php	16	30/30 (100%)	76% (29/38)

Uncovered lines

• framework/captcha/CaptchaAction.php:258 is the auto-detection branch in renderImage(). The tests pin imageLibrary to gd to keep image rendering deterministic across environments.
• framework/captcha/CaptchaAction.php:263, 333-361 are the imagick branch. It is not covered because these tests do not require the imagick extension.
• framework/captcha/CaptchaAction.php:320 is only reachable on PHP < 8.0, while the test run uses PHP 8+.

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.07%. Comparing base (068c5a4) to head (90004fd).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20787      +/-   ##
============================================
+ Coverage     78.62%   79.07%   +0.44%     
  Complexity    11566    11566              
============================================
  Files           376      376              
  Lines         30748    30748              
============================================
+ Hits          24176    24314     +138     
+ Misses         6572     6434     -138     

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