test: add dynamic fee calculation coverage

[afeezorobsco-cyber]

##close #1841

Summary

Adds unit-test coverage for dynamic fee calculations in the Mobile Money ↔ Stellar Bridge.

Changes

• Added coverage for normal dynamic fee calculations.
• Added boundary and amount-based fee tests.
• Covered rounding and precision behavior.
• Covered invalid/edge-case inputs where applicable.
• Added regression coverage for existing expected fee behavior.
• Uses mocked execution without external network dependencies.

Testing

The tests cover:

• normal transaction amounts
• fee boundaries/tiers
• minimum and larger amounts
• rounding behavior
• invalid/edge-case inputs
• maximum supported values where applicable

Validation

[PASTE TEST COMMAND]
[PASTE LINT COMMAND]
[PASTE CLIPPY COMMAND]
[PASTE FORMAT COMMAND]

[drips-wave]

@afeezorobsco-cyber Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[afeezorobsco-cyber]

@sublime247 pls review and merge