Update name to InMemorySaver (#2044)

(Keep MemorySaver around for backwards compatibility)

"MemorySaver" is ambiguous: is it saving memories? Where is it saving
memories to?

InMemorySaver aligns naming InMemoryStore as well as similar LangChain
objects (InMemoryVectorStore, etc.)

libs/langgraph/tests/memory_assert.py

@@ -15,7 +15,7 @@ from langgraph.checkpoint.base import (
     SerializerProtocol,
     copy_checkpoint,
 )
-from langgraph.checkpoint.memory import MemorySaver, PersistentDict
+from langgraph.checkpoint.memory import InMemorySaver, PersistentDict
 
 
 class NoopSerializer(SerializerProtocol):
@@ -26,7 +26,7 @@ class NoopSerializer(SerializerProtocol):
         return "type", obj
 
 
-class MemorySaverAssertImmutable(MemorySaver):
+class MemorySaverAssertImmutable(InMemorySaver):
     storage_for_copies: defaultdict[str, dict[str, dict[str, Checkpoint]]]
 
     def __init__(
@@ -71,7 +71,7 @@ class MemorySaverAssertImmutable(MemorySaver):
         return super().put(config, checkpoint, metadata, new_versions)
 
 
-class MemorySaverAssertCheckpointMetadata(MemorySaver):
+class MemorySaverAssertCheckpointMetadata(InMemorySaver):
     """This custom checkpointer is for verifying that a run's configurable
     fields are merged with the previous checkpoint config for each step in
     the run. This is the desired behavior. Because the checkpointer's (a)put()
@@ -126,7 +126,7 @@ class MemorySaverAssertCheckpointMetadata(MemorySaver):
         )
 
 
-class MemorySaverNoPending(MemorySaver):
+class MemorySaverNoPending(InMemorySaver):
     def get_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
         result = super().get_tuple(config)
         if result: