Add standard error response formatting helper for Fastify exception handling

[Cjay-Cyber-2]

Description

Create a standardized error response formatter utility to ensure all Fastify error replies return a consistent JSON structure containing timestamp, error code, message, and request ID.

Context & Requirements

Consistent error structures improve developer experience for frontend integration (mergepay-web). Fastify's setErrorHandler can be leveraged to format unhandled exceptions and validation errors uniformly.

Acceptance Criteria

• Implement or refine setErrorHandler in the Fastify server configuration.
• Ensure Zod validation errors return formatted 400 responses with detailed issue lists.
• Ensure internal server errors return structured 500 responses without leaking stack traces in production.

Implementation Guidance

• Likely files to touch: src/index.ts or error handling middleware modules.
• Integrate with the request ID propagation feature already established in the codebase.

Testing & Validation

• Add unit or integration tests verifying error response payloads for invalid requests.
• Run npm test.

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

[Glittersup]

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

Hi! I’m interested in this issue. I can trace the existing implementation, identify the appropriate integration points, and implement the change with minimal disruption to the surrounding code. I’ll also cover the relevant tests and edge cases before submitting.

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

[DooseWayo]

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

Can I work on this task?

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