Add success and error-path tests for notifications

[mikewheeleer]

Test notifications

Description

notifications lacks full success/error coverage. This issue adds it.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-backend only.
• Add tests covering notifications success plus the main error paths (validation, not-found, conflict).
• Deterministic; no real network.
• Note any defect uncovered.

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/notifications-91-paths
• 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: success, validation, not-found, conflict.
• Include the full test output in the PR description.

Example commit message

test(notifications): cover success and error paths

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.