test: Unit tests for wallet/fidelity.rs

[Shourya742]

Issue Description:
This issue highlights the need for unit tests in our wallet/fidelity.rs file. Currently, several methods and functions in this file are untested, which could lead to potential issues in the future.

Objective:
Write unit tests for each method and associated function in the wallet/fidelity.rs file.

Details:

• Cover all methods and functions with appropriate test cases.
• Ensure tests validate functionality, handle edge cases, and check error conditions.
• Integrate the tests into our existing test suite.

[wthrajat]

After discussion with @rajarshimaitra, most of the methods are now covered by the PR #90 (in both UTs and ITs)
Some methods like update certificate expiry, find the most valued bond, etc remains which should be potentially covered in integration tests (issue for future).
This can be closed.