<system> umask` / `--umask` is not applied to worker-created files under a foreground supervisor

Is your feature request related to a problem? Please describe.

Follow-up from #5390, and related to #4816

Both --umask and <system> umask set the process umask, but the effect on worker-created files (e.g. file buffer chunks) depends on the run mode:

• --no-supervisor (standalone): applied — Fluentd calls File.umask directly in the worker.
• Supervised + --daemon (daemonize, e.g. fluent-package): applied — ServerEngine sets the umask before spawning workers, so workers inherit it.
• Supervised without --daemon (foreground): not applied — ServerEngine's non-daemonize path doesn't call File.umask, and Fluentd doesn't pass worker_chumask, so workers keep the inherited (shell) umask.

Verified on all three modes: a file buffer chunk created with umask 0077 is 0600 under --no-supervisor and --daemon, but stays 0644 when running fluentd -c fluent.conf in the foreground.

This is existing behavior, not a regression from #5390. Opening this to decide whether to close the gap (e.g. by passing worker_chumask to ServerEngine so foreground workers honor the umask too) or to document it as a known limitation of the run mode.

Describe the solution you'd like

Make --umask / <system> umask also apply to worker-created files under a foreground supervisor (e.g. by passing worker_chumask to ServerEngine so workers set their umask explicitly rather than relying on inheritance)

Describe alternatives you've considered

If the current behavior is intentional, document the per-mode applicability instead, so foreground-supervisor users don't expect it to affect buffer file permissions.

Additional context

No response