Fix JWT encoding, null safety, and code quality issues

[andrei-m-code]

Summary

• Fix Base64URL encoding in both ApnSender and FirebaseSender — was using plain Base64 instead of URL-safe Base64 (RFC 7617: replace +→-, /→_, strip = padding). This could cause malformed JWTs and authentication failures with Apple/Google.
• Fix NullReferenceException in ApnSender error handling — Deserialize<ApnsError>(content).Reason now uses null-conditional operator to handle unexpected error formats from Apple.
• Fix static token cache key collision — cache key now includes P8PrivateKeyId so multiple ApnSender instances with the same bundle ID but different credentials don't share tokens.
• Add device token validation in ApnSender.SendAsync() — throws ArgumentException on null/empty tokens instead of sending invalid requests to Apple.
• Propagate CancellationToken through FirebaseSender.GetJwtTokenAsync() — the token was accepted by SendAsync but not passed to internal HTTP calls.
• Remove infinite loop in CorePush.Tester and log the response.
• Remove unused System.IdentityModel.Tokens.Jwt dependency from CorePush.Tester.

Test plan

• Verify solution builds cleanly (dotnet build CorePush.sln)
• Test APN push notification sending with valid credentials
• Test Firebase push notification sending with valid credentials
• Verify cancellation works in Firebase sender

[andrei-m-code]

Superseded by a more comprehensive PR that also removes BouncyCastle and upgrades to .NET 10