fix: suppress dead-letter logging during a terminating shutdown when disabled (#3256)

[He-Pin]

Motivation

When pekko.log-dead-letters-during-shutdown = off (which is the default,
reference.conf:63), dead letters were still logged throughout the entire
CoordinatedShutdown sequence. DeadLetterListener had no shutdown awareness
and was only stopped at the very end of shutdown (finalTerminate), so every
dead letter produced while the system was terminating — potentially for the
whole (and sometimes lengthy) shutdown — was logged unconditionally. The
setting was effectively ignored until the very last moment.

Fixes #3256.

Modification

DeadLetterListener now suppresses dead-letter logging while a coordinated
shutdown that actually terminates this ActorSystem is in progress, when
log-dead-letters-during-shutdown is disabled.

Crucially, whether the shutdown terminates the system is taken from the
effective per-reason configuration, using the exact same resolution that
CoordinatedShutdown itself uses
(CoordinatedShutdown.confWithOverrides(conf, shutdownReason()).getBoolean("terminate-actor-system")).
This matters because terminate-actor-system can be overridden per shutdown
reason. A coordinated shutdown that does not terminate the system (globally
via terminate-actor-system = off, or only for a specific reason via
reason-overrides) leaves a shutdown reason set on a still-running system —
that must not suppress logging, otherwise dead-letter logging would be
silently disabled for the remaining lifetime of the running system.

The CoordinatedShutdown extension reference is cached and the effective
decision is memoized (computed at most once — the shutdown reason is set once
and never changes), so the per-dead-letter path stays allocation-free and cheap
(a volatile read of the shutdown reason during normal operation, no extension
lookup).

Result

• Dead letters are no longer logged during a terminating shutdown when
  log-dead-letters-during-shutdown = off — the setting is now honored for the
  whole shutdown, not just after the listener is finally stopped.
• Logging continues normally for a non-terminating coordinated shutdown
  (globally or per shutdown reason) and during normal operation.
• All new members are private; no public API or binary-compatibility
  change — actor / mimaReportBinaryIssues is clean.

Tests

• sbt "actor-tests/testOnly org.apache.pekko.event.DeadLetterListenerShutdownSpec"
  → 4 passed, plus DeadLetterSupressionSpec and DeadLetterSuspensionSpec
  (no regression).
• Directional tests (a paused first CoordinatedShutdown phase keeps the
  listener alive mid-shutdown) that fail before the fix:
  • a terminating shutdown must suppress the dead letter;
  • a non-terminating shutdown (global terminate-actor-system = off) must keep
    logging;
  • a non-terminating shutdown via a per-reason override must keep logging
    (this one fails against an earlier base-setting-only gate);
  • logging must continue even after a non-terminating run completes.
• sbt "actor/mimaReportBinaryIssues" → no binary issues.

References

Fixes #3256

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This is an original contribution to Apache Pekko, made under the Apache License
2.0 (i.e. the changes are now Apache licensed). No third-party or Akka-derived
code is included.