chore(delta-channel): remove supports_delta_channels flag

Rely on the runtime raise in DeltaChannel.from_checkpoint() instead of
a compile-time boolean flag. Savers that assemble DeltaChainValue inside
_load_blobs work transparently; savers that don't will pass through a raw
DeltaValue and hit a clear ValueError on first reload.

Removes: BaseCheckpointSaver.supports_delta_channels, the attribute on
InMemorySaver / PostgresSaver / AsyncPostgresSaver, the compile-time
UserWarning in StateGraph.compile(), and the associated test.

libs/checkpoint-postgres/langgraph/checkpoint/postgres/__init__.py

@@ -32,7 +32,6 @@ Conn = _internal.Conn  # For backward compatibility
 class PostgresSaver(BasePostgresSaver):
     """Checkpointer that stores checkpoints in a Postgres database."""
 
-    supports_delta_channels: bool = True
     lock: threading.RLock
 
     def __init__(

libs/checkpoint-postgres/langgraph/checkpoint/postgres/aio.py

@@ -34,7 +34,6 @@ Conn = _ainternal.Conn  # For backward compatibility
 class AsyncPostgresSaver(BasePostgresSaver):
     """Asynchronous checkpointer that stores checkpoints in a Postgres database."""
 
-    supports_delta_channels: bool = True
     lock: asyncio.Lock
 
     def __init__(