test: add concurrent user load testing for contract (#852)

[Stephan-Thomas]

Pull Request Template

Linked Issue

Closes #852

PR Title

test(contract): add concurrent user load testing

This PR title follows the Conventional Commits format as required by CI. See PR Title Conventions for details.

What Changed

Added concurrent load tests for the contract to evaluate its behavior under high user activity.

The tests simulate 100+ concurrent job postings and concurrent accept_job attempts against the same job. The test suite tracks transaction success rates and captures failure cases to identify potential bottlenecks and concurrency-related failure modes.

The results provide a baseline for contract performance and behavior under concurrent user load.

Validation

• I referenced the related issue in this PR.
• Contract checks pass (soroban contract build and cargo test in contracts/escrow) if contract code changed.
• Frontend checks/build pass for changed frontend files.
• I included screenshots or short clips for UI changes (or noted N/A).
• I verified the UI changes in the preview deployment.

Additional Notes

• Load tests cover 100+ concurrent job posting operations.
• Concurrent accept_job attempts against the same job are tested to evaluate contention and contract behavior.
• Transaction success rates and failure modes are captured for performance analysis.
• The tests are focused on identifying concurrency-related bottlenecks and reliability issues.
• No frontend changes are included in this PR, so frontend and preview deployment checks are not applicable.
• Performance characteristics observed during the load tests can be used as a baseline for future optimization and regression testing.

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@Stephan-Thomas Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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