Add ArrayExpressionTest

[WarLikeLaux]

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

What does this PR do?

Adds ArrayExpressionTest - unit tests for yii\db\ArrayExpression.

Before

Tests: 0
Methods: 0% (0/10), Lines: 0% (0/19)
MSI: 0%

After

Tests: 28, Assertions: 47
Methods: 100% (10/10), Lines: 100% (19/19)
MSI: 100% (18/18 mutants killed)

Tests

ArrayExpressionTest (28 tests, 47 assertions):

• Constructor: default values, type/dimension params, nested ArrayExpression unwrap, QueryInterface passthrough
• ArrayAccess/Countable/IteratorAggregate interface contract
• getIterator() exception on QueryInterface, null value handling

Escaped mutants (all equivalent)

None. All 18 mutants killed.

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.62%. Comparing base (068c5a4) to head (ce71233).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #20766   +/-   ##
=========================================
  Coverage     78.62%   78.62%           
  Complexity    11566    11566           
=========================================
  Files           376      376           
  Lines         30748    30748           
=========================================
  Hits          24176    24176           
  Misses         6572     6572           

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