Updated CustomPolicyProvider to DotNet10 Style Project

[samabos]

Updated CustomPolicyProvider to DotNet10 Style Project

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Summary of the changes (Less than 80 chars)

Description

• Refactored Program.cs to use top-level program style in line with .NET 10.
• Removed Startup.cs.
• Updated CustomPolicyProviderTest to ensure tests can use Program.cs.

Fixes #61462

[dotnet-policy-service]

Thanks for your PR, @@samabos. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.