Add helper for building a test HTTP request with all required auth headers pre-populated

[chiscookeke11]

Summary

• Added a reusable test utility for generating authenticated Supertest requests with valid Stellar wallet signatures.
• Introduced buildAuthHeaders() and buildAuthRequest() to reduce duplication across integration tests for mutating endpoints.
• Supported POST, PUT, PATCH, and DELETE requests with automatic authentication headers.
• Allowed injection of a lightweight Express app to simplify test setup and avoid compiling full application dependencies.
• Added unit tests verifying authenticated POST and DELETE requests successfully pass the requireStellarSignature middleware.

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate when schema or generated types changed

Checklist

• Linked issue or backlog item
• No secrets or live credentials added
• Docs updated if setup or env changed
• Change is scoped to one problem

Closes #552

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