Fix queryScalar() state mutation by using clone

[darkspock]

Summary

Query::queryScalar() manually saves and restores select, orderBy, limit, and offset properties around createCommand(). This is fragile:

• If createCommand() throws an exception, the original state is never restored
• Not safe if properties hold references used elsewhere
• Inconsistent with the else branch (line 856) which already creates a new Query instance

Before

$select = $this->select;
$order = $this->orderBy;
$limit = $this->limit;
$offset = $this->offset;

$this->select = [$selectExpression];
$this->orderBy = [];
$this->limit = null;
$this->offset = null;

$command = $this->createCommand();

$this->select = $select;
$this->orderBy = $order;
$this->limit = $limit;
$this->offset = $offset;

return $command->queryScalar();

After

$query = clone $this;
$query->select = [$selectExpression];
$query->orderBy = [];
$query->limit = null;
$query->offset = null;

return $query->createCommand()->queryScalar();

The clone approach is already the established pattern in this codebase (see withTypecasting()).

Risk

Low — the clone produces identical SQL output. The only behavioral change is that the original Query object is no longer temporarily mutated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.62%. Comparing base (dffa745) to head (7e710c5).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1172      +/-   ##
============================================
- Coverage     98.62%   98.62%   -0.01%     
  Complexity     1645     1645              
============================================
  Files           120      120              
  Lines          4292     4284       -8     
============================================
- Hits           4233     4225       -8     
  Misses           59       59              

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[Tigrov]

Needs to add a line to changelog.