fix: java shutdown sequence fixing

[nicklasl]

Summary

Refactors Java provider shutdown lifecycle to properly manage resources:

• Factories don't manage shutdown: ChannelFactory and new HttpClientFactory create resources but delegates shutdown to callers
• Improved shutdown sequence: Provider now properly terminates executors, channels, and HTTP clients with timeouts
• Sync log writes on shutdown: Adds writeSync() to flush logs before termination
• Comprehensive tests: New integration tests and gRPC flag logger tests with in-process server

Fixes resource leaks and ensures clean shutdown.