Updated CustomPolicyProvider to DotNet10 Style Project

[samabos]

Updated CustomPolicyProvider to DotNet10 Style Project

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

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.

[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
To make sure no conflicting changes have occurred, please rerun validation before merging. You can do this by leaving an /azp run comment here (requires commit rights), or by simply closing and reopening.

[samabos]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 61474 in repo dotnet/aspnetcore