Make stdio logging fail-soft on write errors

[dineshadhi]

Foundations currently wraps terminal/stderr logging drains with Fuse before passing them to slog_async. If the underlying stdout/stderr stream returns an I/O error such as BrokenPipe, Fuse turns that error into a panic in the async logging worker.

This is unsafe for services whose stdout/stderr is captured by systemd and journald: if the journald stream is interrupted, normal logging can crash the service.

Handle terminal and stderr output errors with ignore_res() instead, so stdio logging failures do not panic the async logging worker. File logging keeps the existing fused behavior.

Add a regression test proving a failing async stdio drain keeps processing subsequent records instead of terminating after the first write error.

[TheJokr]

The use of fuse() for our loggers is intentional - keeping a service running without log output is bad for observability. That being said, I understand there is a use case for this. I'm happy to merge a PR that makes this configurable:

• Add a field ignore_io_errors: bool to LoggingSettings
• Wrap the AsyncDrain construction in a new function that takes drain: impl Drain<...> and settings: &LoggingSettings
• Apply fuse() or ignore_res() depending on the setting there
• Use that function for the output types that are currently fused (terminal/stderr/file)