Add Zod schema validation for expense creation request payloads

[Cjay-Cyber-2]

Description

Implement a robust Zod validation schema for expense creation requests in the backend API to ensure all incoming payload fields (such as description, amount, currency, and participants) are correctly typed and formatted before processing.

Context & Requirements

The Mergepay backend uses Fastify and Zod for input validation. Currently, some expense endpoints lack comprehensive request body validation, risking malformed data entering the database or settlement services. We need a dedicated Zod schema for expense creation matching the client types in mergepay-web.

Acceptance Criteria

• Create a Zod validation schema for expense creation in src/schemas/expense.schema.ts.
• Validate description (non-empty string), amount (positive number/string representation), currency (XLM or USDC), and participant shares.
• Integrate the schema into the expense creation route handler.
• Add unit tests verifying successful validation and proper rejection of invalid payloads.

Implementation Guidance

• Likely files to touch: src/schemas/expense.schema.ts, route definitions under src/routes/.
• Follow existing Zod validation patterns found across other route handlers in the codebase.

Testing & Validation

• Run npm test to ensure all existing and new unit tests pass successfully.

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

[devNuel]

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

Hello, I’ve carefully reviewed the issue description and have a clear understanding of the expected outcome. I’d be delighted to take this on and contribute to resolving it effectively.

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[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.

[melanindebbie]

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

Hi maintainer, I can resolve this issue immediately. Please assign to me

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

[drips-wave]

Congratulations, @melanindebbie! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @melanindebbie: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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