fix(actor): harden lifecycle, supervision, Redis mailbox, and add comprehensive tests

- Fix spawn() zombie cell: clean up registry on start() failure
- Fix shutdown(): cancel + await tasks that exceed graceful timeout
- Fix _shutdown(): await mailbox.close() to release backend resources
- Fix escalate directive: stop failing child before propagating to grandparent
- Fix RedisMailbox.put(): wrap Redis errors in try/except, return False on failure
- Fix retry.py: replace assert with proper raise for last_exc
- Add put_batch() to Mailbox abstraction for single-roundtrip bulk enqueue
- Add RedisMailbox.put_batch() with atomic Lua script for bounded queues
- Add MailboxFullError exception type for semantic backpressure handling
- Add redis>=7.4.0 dependency with public PyPI sources in uv.lock

Tests added (31 total, up from 27):
- test_middleware_on_restart_hook: verifies middleware.on_restart() on supervision restart
- test_ask_propagates_actor_exception: ask() re-raises original exception type
- test_ask_propagates_exception_while_supervised: exception propagates; root actor survives
- test_ask_timeout_late_reply_no_exception: late reply after timeout is silent no-op
- test_actor_backpressure.py: MailboxFullError + dead letter on full mailbox
- test_actor_retry.py: ask_with_retry with exponential backoff
- test_mailbox_redis.py: RedisMailbox put/get/batch/close
- bench_actor_redis.py: RedisMailbox throughput benchmarks

backend/packages/harness/deerflow/actor/__init__.py

@@ -19,7 +19,8 @@ Usage::
 from .actor import Actor, ActorContext
 from .mailbox import Mailbox, MemoryMailbox
 from .middleware import Middleware
-from .ref import ActorRef, ReplyChannel
+from .ref import ActorRef, MailboxFullError, ReplyChannel
+from .retry import IdempotentActorMixin, IdempotencyStore, RetryEnvelope, ask_with_retry
 from .supervision import AllForOneStrategy, Directive, OneForOneStrategy, SupervisorStrategy
 from .system import ActorSystem, DeadLetter
 
@@ -32,9 +33,14 @@ __all__ = [
     "DeadLetter",
     "Directive",
     "Mailbox",
+    "MailboxFullError",
     "MemoryMailbox",
     "Middleware",
     "OneForOneStrategy",
     "ReplyChannel",
+    "RetryEnvelope",
     "SupervisorStrategy",
+    "IdempotentActorMixin",
+    "IdempotencyStore",
+    "ask_with_retry",
 ]