[V-PHX-VUL-030] Missing test coverage

[gangov]

Here we list code locations that are not tested in any unit tests:

packages/decimal/src/lib.rs

• [x] * pub fn abs (line 213)
• [x] * pub fn to_string (line 221)
• [x] * fn fmt (line 333)

Impact: Erroneous updates to untested functions may more easily lead to bugs in production.

Recommendation: Add tests to increase code coverage.