fix: PostgreSQL Builder's increment() and decrement() methods not working for numeric columns

[patel-vansh]

Description
As discovered in #10140 in this comment, the PostgreSQL Builder's increment() and decrement() methods were giving errors while incrementing/decrementing numeric columns.

This PR fixes that by changing "to_number({$column}, '9999999') + {$value}" to "CAST({$column} AS numeric) + {$value}", so that both string as well as numeric columns work as expected.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[neznaika0]

Isn't the current test tests/system/Database/Live/IncrementTest.php running for postgresql?
https://github.com/codeigniter4/CodeIgniter4/actions/runs/25487825930/job/74789224632?pr=10172
why do we need a duplicate test?

[patel-vansh]

Isn't the current test tests/system/Database/Live/IncrementTest.php running for postgresql? https://github.com/codeigniter4/CodeIgniter4/actions/runs/25487825930/job/74789224632?pr=10172 why do we need a duplicate test?

Well, yes, I deliberately added that new file because I thought that testing increment for numeric columns was explicit to PostgreSQL, but you're right, I will add that to the normal file only so that other DB builders also get tested with numeric columns.