fix: encapsulate express to string conversion

[coryhh]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 12.12121% with 87 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@4e0818e). Click here to learn what that means.

Files	Patch %	Lines
.../sqlparse/select/ArexExpressionVisitorAdapter.java	5.68%	83 Missing ⚠️
...rse/sqlparse/select/utils/ExpressionExtractor.java	57.14%	2 Missing and 1 partial ⚠️
...e/sqlparse/select/ArexItemsListVisitorAdapter.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage        ?   62.34%           
  Complexity      ?      952           
=======================================
  Files           ?      116           
  Lines           ?     4443           
  Branches        ?      715           
=======================================
  Hits            ?     2770           
  Misses          ?     1334           
  Partials        ?      339           

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