Extract the shared metrics handler validation into a reusable helper

[mikewheeleer]

Deduplicate metrics validation

Description

The metrics handlers repeat similar validation preambles inline. This issue extracts a shared, tested validation helper.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-backend only.
• Extract the repeated metrics validation into a single helper consumed by each handler.
• Behaviour unchanged; same rejections and codes.
• No new dependencies.

Suggested execution

• Fork the repo and create a branch
• git checkout -b refactor/metrics-01-validation-helper
• Implement changes
  • Write code in: the relevant module.
  • Write comprehensive tests in: cover the new behaviour and edge cases.
• Test and commit

Test and commit

• Run npm run lint, npm test, and npm run build.
• Cover edge cases: each existing rejection still fires identically.
• Include the full test output in the PR description.

Example commit message

refactor(metrics): extract shared validation helper

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the AgentPay community on Discord: https://discord.gg/eXvRKkgcv
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — a 5-star rating is much appreciated.

[thlpkee20-wq]

Please consider assigning this issue to me, as I’m prepared to contribute wherever it’s needed, whether that involves protocol logic, interface creation, service-layer development, visual implementation, or cross-component coordination.

[grantfox-oss]

🦊 GrantFox — @thlpkee20-wq has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #391)
2. Your PR will be reviewed by the Agentpay-Org maintainers

Good luck! Track your progress on GrantFox.