[2026 Testing] Add property-based tests for path payment amount invariants

[Nanle-code]

Objective

Use property tests to assert amount conservation and rounding invariants for path payments.

Target area

property test suites

Acceptance criteria

• The objective is implemented with clear handling for invalid input, unsupported environments, and failure paths.
• Automated tests cover the primary flow, at least one boundary case, and at least one failure case.
• User-facing documentation or developer guidance is updated, including compatibility, security, or migration notes where applicable.
• Pull requests that address this issue must keep the branch free of merge conflicts with the target branch.
• All continuous integration checks must pass before the pull request may be merged; failing, pending, or skipped required checks are not acceptable for merge.

Pull request requirements

• All continuous integration checks must pass before a pull request may be merged.
• The branch must be free of merge conflicts with the target branch at the time of merge.
• Do not request merge with failing workflows, skipped required checks, or unresolved conflicts.

[collins-uzu]

@collins-uzu has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello, I have reviewed the issue and understand what needs to be done.

I would appreciate being assigned to it.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @collins-uzu to this issue.

[timmyp10111]

@timmyp10111 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Please assign this issue to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @timmyp10111 to this issue.

[drips-wave]

Congratulations, @timmyp10111! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @timmyp10111: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊