Consolidate Neptune and RDS backends (#7925)

Despite being documented as two distinct services (with two distinct boto3 clients), the Neptune and RDS services share a single AWS backend (RDS). This is reflected in the botocore service definition for Neptune, which lists `rds` as its endpoint. Calling the `DescribeDBClusters` API method with either the `neptune` or `rds` boto3 client results in the exact same list of clusters--no filtering is done on the AWS backend. Neptune clusters are commingled with Aurora clusters.

moto/backends.py

@@ -94,7 +94,6 @@
     from moto.meteringmarketplace.models import MeteringMarketplaceBackend
     from moto.moto_api._internal.models import MotoAPIBackend
     from moto.mq.models import MQBackend
-    from moto.neptune.models import NeptuneBackend
     from moto.networkmanager.models import NetworkManagerBackend
     from moto.opensearch.models import OpenSearchServiceBackend
     from moto.opensearchserverless.models import OpenSearchServiceServerlessBackend
@@ -147,13 +146,18 @@
     from moto.xray.models import XRayBackend
 
 
-ALT_SERVICE_NAMES = {"lambda": "awslambda", "moto_api": "moto_api._internal"}
+ALT_SERVICE_NAMES = {
+    "lambda": "awslambda",
+    "moto_api": "moto_api._internal",
+    "neptune": "rds",
+}
 ALT_BACKEND_NAMES = {
     "moto_api._internal": "moto_api",
     "awslambda": "lambda",
     "awslambda_simple": "lambda_simple",
     "dynamodb_v20111205": "dynamodb",
     "elasticbeanstalk": "eb",
+    "neptune": "rds",
 }
 
 
@@ -556,7 +560,7 @@ def get_backend(name: "Literal['moto_api']") -> "BackendDict[MotoAPIBackend]": .
 @overload
 def get_backend(name: "Literal['mq']") -> "BackendDict[MQBackend]": ...
 @overload
-def get_backend(name: "Literal['neptune']") -> "BackendDict[NeptuneBackend]": ...
+def get_backend(name: "Literal['neptune']") -> "BackendDict[RDSBackend]": ...
 @overload
 def get_backend(
     name: "Literal['networkmanager']",

moto/neptune/README.md

@@ -0,0 +1,3 @@
+Neptune service is handled by the RDS service backend located [here](../rds).
+
+Neptune service has a dedicated test suite located [here](../../tests/test_neptune).