[13.x] fix(postgres): whereDate and whereTime crash when $column is an Expression

[ahawlitschek]

The ->whereDate and ->whereTime functions are typed to accept Expression or string as first parameter ($column). Since Laravel 12, both underlying functions in PostgresGrammar checks whether the $column is a JSON selector using the isJsonSelector($value) function defined in the base grammar. This function uses str_contains and causes a runtime exception if it receives something other than string. Currently, the function is called with the origin value of the $column parameter which in some cases may be Expression.

This adds an is_string check to only call isJsonSelector if the type of the column is a string and also adds tests which check, that using Expression in whereDate and whereTime produces proper SQL.

I discussed several ways to fix this with my colleagues. Swapping $this->isJsonSelector($where['column']) with $this->isJsonSelector($column) would also fix it. But in my opinion an Expression is semantically no json selector and the developer is responsible for ensuring a valid query if DB::raw() is used.
Changing isJsonSelector to also handle Expression seems too broad.
Another approach would be to always add parentheses within the return statement, since the parentheses are necessary because of the cast. But then more tests need to be adjusted and the created SQL will contain unnecessary parentheses most of the time.