feat: adopt AndreGoepel.Marten.Testing

[andregoepel]

Summary

Replaces the container-lifecycle code in MartenFixture.cs with the newly published AndreGoepel.Marten.Testing 1.0.0, using its documented extensibility hooks:

• ConfigureStore → InitializeIdentity()
• OnStoreInitializedAsync → resolves ISettingsStore through the public AddMartenConfiguration() DI registration
• ResetAsync override → base.ResetAsync + DeleteAllEventDataAsync

IntegrationCollection.cs needed no changes — per the package's documented pattern, each consumer keeps its own [CollectionDefinition] bound to its own fixture subclass. Removed the now-unused direct Testcontainers.PostgreSql reference.

Verification

• dotnet csharpier format . clean
• dotnet build — 0 warnings, 0 errors
• dotnet test — 92/92 integration tests passing against real Testcontainers Postgres
• dotnet list package --vulnerable --include-transitive — clean

Closes #154