Add comprehensive unit tests for Stellar memo format validation utility

[Cjay-Cyber-2]

Description

Add dedicated unit tests for the shared Stellar memo format validation utility to ensure memos adhere strictly to the MP:<code> structure required by the settlement engine.

Context & Requirements

Stellar transactions rely on standardized memos to link on-chain payments to specific expenses (MP:<code>). While validation functions exist, test coverage for edge cases (such as missing prefixes, invalid code lengths, or unexpected characters) needs reinforcement.

Acceptance Criteria

• Add a test file src/utils/__tests__/memo.test.ts (or equivalent test directory).
• Test valid memos (MP:AB123) and invalid memos (empty string, missing MP: prefix, lowercase letters, special characters).
• Ensure 100% pass rate when running Vitest.

Implementation Guidance

• Likely files to touch: src/utils/ (locate existing memo helper), new test file.
• Use Vitest assertions matching the project's testing setup.

Testing & Validation

• Execute npm test and verify that the new memo validation test suite runs and passes cleanly.

Submission Guidelines

• Open a Pull Request referencing Closes #<issue-number>.
• Adhere to TypeScript strict rules and existing codebase formatting standards.

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Wave complexity: Trivial

[Longcrypt]

@Longcrypt has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I’d like to work on this project

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Longcrypt to this issue.

[pelumixxril]

@pelumixxril has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer,I would like request for the access on resolving this issue sir,got few years experience on how to get such done sir,kindly do well to assign this issue to me sir,thank you sir.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @pelumixxril to this issue.

[chiemezie1]

@chiemezie1 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi, I’ve reviewed the issue and understand the requirements. I have the skills needed to work on it and would be happy to take it up. Please assign it to me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @chiemezie1 to this issue.