Backend tests warn about Pomelo EF Core package version outside dependency constraint

[simplerjiang]

• Found by: local backend tests.
• Impact: Repeated NU1608 warnings may indicate unsupported EF provider/runtime pairing.
• Repro:
  1. Run dotnet test .\backend\SimplerJiangAiAgent.Api.Tests\SimplerJiangAiAgent.Api.Tests.csproj --no-restore.
• Expected: Dependency graph has no NU1608 warnings.
• Actual: Pomelo.EntityFrameworkCore.MySql 8.0.2 requires Microsoft.EntityFrameworkCore.Relational <= 8.0.999, but 9.0.4 is resolved.
• Claim: Assign backend dependency owner.
• Repair: Align Pomelo/EF versions or document why the warning is acceptable.
• Verification: dotnet test no longer prints NU1608 for Pomelo/EF, or dependency decision is captured.