Add unit tests for web utilities

[WarLikeLaux]

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

What does this PR do?

Adds unit tests for 4 web utility classes that had no dedicated test coverage: Cookie, JsonParser, SessionIterator, JsExpression.

Before

Tests: 0, Assertions: 0
Methods: 0% (0/10), Lines: 0% (0/20)
MSI: 0% (0/32 mutants killed)

After

Tests: 37, Assertions: 62
Methods: 100% (10/10), Lines: 100% (20/20)
MSI: 97% (28/32 mutants killed, 3 timed out, 1 equivalent)

Escaped mutants (all equivalent)

JsExpression.php (1): MethodCallRemoval - L38, parent::__construct($config) removal with empty config is a no-op.

---

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.65%. Comparing base (379b296) to head (1d83278).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20771      +/-   ##
============================================
+ Coverage     78.56%   78.65%   +0.08%     
  Complexity    11568    11568              
============================================
  Files           376      376              
  Lines         30750    30750              
============================================
+ Hits          24160    24185      +25     
+ Misses         6590     6565      -25     

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