Add boundary tests for the authorization logic

[mikewheeleer]

Cover authorization boundaries

Description

The authorization logic is thinly tested at its boundaries. This issue adds focused boundary and rejection tests.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-contracts only.
• Add tests for the accept/reject boundaries of authorization, asserting exact typed error codes.
• Use the test-utils helpers; assert events where the flow emits them.
• Do not change contract logic unless a defect is found (note it).

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/authorization-01-boundaries
• 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 cargo fmt, cargo clippy --all-targets -- -D warnings, and cargo test.
• Cover edge cases: exactly-at boundary, one over, unauthorized caller.
• Include the full test output in the PR description.

Example commit message

test(authorization): cover boundaries and rejections

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.