Fix zero-decimal currency formatting

[troioi-vn]

Summary

This fixes formatting for zero-decimal currencies such as VND.

When decimal_position is 0, the currency format should not include a decimal separator. The previous logic always appended the decimal sign, which could lead to invalid formatting assumptions downstream.

Changes

• omit the decimal separator when decimal_position is 0
• add regression tests for zero-decimal currency formatting

Verification

• added test coverage for decimal_position = 0
• manually sanity-checked the formatting path for a VND-like currency

Notes

I could not run the full test suite locally because the project test setup expects a DB driver that is not available in my current environment.