issue #9807 refactored syntax for better understanding

[AkshatHotCode]

This pull request addresses readability and uniqueness issues in the CoalesceAnalyzer class. The primary focus is on improving the temporary variable name used in the transformation of the coalesce operator into a fake ternary. The changes aim to make the variable name more descriptive and less likely to conflict with existing variables in the scope.

Key Changes:

Temporary Variable Name: Replaced the existing temporary variable name with a more descriptive and unique name: $<psalm_coalesce_temp_var>. Added a type hint to the temporary variable: array<array-key, mixed>. Context:

The previous naming convention ($XXX) could potentially clash with existing variables in the scope, leading to bugs. The new naming convention improves clarity and ensures uniqueness. Additionally, the type hint provides information about the expected type of the temporary variable.

These changes should enhance the readability of the code and mitigate potential issues related to variable naming conflicts in the CoalesceAnalyzer.

fixes #9807