Add Polly retry logic to fix flaky email verification integration test

[oveldman]

Summary

The ThenTheAdministratorShouldHaveReceivedAnEmailWithSubject step was failing intermittently because the Mailpit API was polled once immediately after sending feedback, before the mail had been delivered. Replaced the single request with a Polly retry pipeline that retries up to 10 times (200ms delay) until the expected email subject appears.

Changes

• Add Polly.Core 8.7.0 to Directory.Packages.props and Api.IntegrationTests.csproj
• Rewrite the email-check step in CorrespondencesSteps to use a ResiliencePipeline with RetryStrategyOptions — retries until the expected subject is found or 10 attempts are exhausted
• Use the TState overload of ExecuteAsync with a static lambda to avoid closure allocation
• Add Polly to UsefulLinks.md

How to Test

• Run the correspondence integration tests: dotnet test tests/MadWorldEU.Byakko.Controller.Api.IntegrationTests/ -f Correspondences
• Verify the ThenTheAdministratorShouldHaveReceivedAnEmailWithSubject step passes consistently across multiple runs

Checklist

• Code builds without errors
• Changes have been tested locally
• No new warnings introduced

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud