Fix PostgreSQL JSONB subscripting regression with functions

Fix PostgreSQL JSONB subscripting regression with functions

Fixed regression in PostgreSQL dialect where JSONB subscription syntax
would generate incorrect SQL for JSONB-returning functions, causing syntax
errors. The dialect now properly wraps function calls and expressions in
parentheses when using the ``[]`` subscription syntax, generating
``(function_call)[index]`` instead of ``function_call[index]`` to comply
with PostgreSQL syntax requirements.

Fixes: #12778
Change-Id: If1238457e6bba6a933023b26519a41aa5de4dbcd