[monarch] Refactor pickling to use thread-local Rust storage

[zdevito]

Stack from ghstack (oldest at bottom):

• -> [monarch] Refactor pickling to use thread-local Rust storage #2718

This diff refactors Monarch's pickling system by moving from a Python-based
persistent_id/flatten/unflatten approach to a simpler Rust-based
thread-local storage mechanism.

Key Changes:

1. New pickle.rs module - Introduces thread-local ACTIVE_PICKLING_STATE
   storage for tracking out-of-band pickling information during cloudpickle
   operations. Provides PicklingState, PendingMessage, and pickle() function.

2. Simplified PythonMessage - Removed pending_pickle_state field entirely.
   Constructor now takes FrozenBuffer directly instead of Buffer | bytes.

3. Removed mailbox handling from references - The local_state for message
   dispatch changed from itertools.repeat(mailbox) to an empty list. Mailboxes
   are no longer passed through this mechanism.

4. Deleted PendingPickle and PendingPickleState from pytokio.rs - These
   Python-side classes handled deferred pickling via flatten/unflatten.
   Replaced by Rust-side PicklingState.resolve() and PendingMessage.resolve().

5. PyShared now has __reduce__ - Added pickle protocol support directly
   via reduce_shared(). Also optimized block_on to check if value is already
   available before calling into tokio runtime.

6. New cast_unresolved() method - Trait method for casting messages with
   unresolved async values. AsyncActorMesh provides async implementation.

7. Python-side simplifications - Removed helper functions (_is_mailbox,
   _flatten_with_pending_pickle, _pickle), _SingletonActorAdapator class,
   and allow_pending_pickle_mesh() context manager usage.

Benefits:

• Reduced Python overhead: No more Python-side persistent_id callbacks or
  flatten/unflatten traversals during pickling
• Cleaner architecture: Pickling state handled via thread-local Rust storage
  that __reduce__ implementations can access directly
• Simplified message type: PythonMessage no longer carries pending pickle state
• ~200 lines of Python removed, ~175 lines of Rust removed from pytokio.rs

Differential Revision: D92435072