Add NServiceBus 10 Sample: Recoverability Policy Testing (#7597)

* Add Dockerfile for recoverabilitypolicytesting Core_9 sample

- Added multi-stage Dockerfile that runs unit tests during build stage
- Tests validate custom recoverability policy behavior:
  - Messages with MyBusinessTimedOutException are discarded
  - Unrecoverable exceptions go to custom error queue
  - Other exceptions go to default error queue
- Simplified approach for unit testing sample (no console apps or startup script needed)

* Add recoverabilitypolicytesting Core_10 sample for NServiceBus 10

- Updated to target .NET 10 with preview language features
- Updated NServiceBus packages to latest alpha versions:
  - NServiceBus: 10.0.0-alpha.2
  - NServiceBus.Testing: 10.0.0-alpha.3
- Updated Dockerfile for .NET 10 preview with required environment variable
- Added prerelease.txt marker file
- All unit tests pass, validating custom recoverability policy behavior

* Apply C# 12 collection expressions to recoverabilitypolicytesting Core_10

- Use collection expressions for HashSet initialization: [typeof(DivideByZeroException)]
- Use collection expressions for empty collections: [] instead of new HashSet<Type>()
- Use collection expressions for empty byte arrays: [] instead of new byte[0]
- Maintains same functionality with cleaner, more modern syntax
- All tests continue to pass

Author: boblangley