Extract the shared cors handler validation into a reusable helper

[mikewheeleer]

Deduplicate cors validation

Description

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

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-backend only.
• Extract the repeated cors 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/cors-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(cors): 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 StableRoute community on Discord: https://discord.gg/37aCpusvx
• ⭐ 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.