Use FlushWriter to ensure traces are flushed

[SteveL-MSFT]

PR Summary

Still seeing random failures particulary on macOS due to traces not being flushed before dsc exits. Doing a bit more research, it seems that macOS itself caches stdio output and thus you need to explicitly flush. Fix here is to use a custom FlushWriter to flushes on every write.

PR Context

Fix #677

[Copilot]

Pull request overview

This PR addresses intermittent trace-related CI failures (notably on macOS) by ensuring tracing output is flushed promptly before the dsc process exits. It does so by wrapping the tracing subscriber’s writer with a custom writer that flushes after writes.

Changes:

• Added a FlushWriter wrapper implementing std::io::Write that flushes after writing.
• Added a MakeFlushWriter wrapper implementing tracing_subscriber::fmt::MakeWriter to produce FlushWriter instances.
• Updated enable_tracing to use the flushing writer for both the default and configured tracing fmt layers.

[SteveL-MSFT]

macOS test still failed, it appears that the flushing is only when using PS based classes, so making change in PS adapter