[firestore-emulator: datastore-mode] reserve_sequential breaks with negative keys

### [REQUIRED] Environment info

firebase-tools: Using the gcloud cli version 511.0.0, firestore emulator: 1.19.9
Platform: macOS (arm), python client library

### [REQUIRED] Test case

```python
import os

from google.cloud import datastore

def main():
PORT = 10901
os.environ["DATASTORE_EMULATOR_HOST"] = "127.0.0.1:%s" % PORT
os.environ["DATASTORE_PROJECT_ID"] = "test"

def get_new_client():
return datastore.Client(
project=os.environ.get("GCLOUDC_PROJECT_ID", "test"),
namespace=None,
_http=None,
)

client = get_new_client()
kind = "a_kind"
key = client.key(kind, -1)

client.reserve_ids_sequential(key, 1)

if __name__ == "__main__":
main()
```

### [REQUIRED] Steps to reproduce

1. Start the emulator with gcloud emulators firestore start --host-port=127.0.0.1:10901 --database-mode=datastore-mode
2. Install the python deps: pip install google-cloud-datastore==2.19.0
3. Run the above python script

### [REQUIRED] Expected behavior

Runs successfully, without errors

### [REQUIRED] Actual behavior

Command fails with

```
Traceback (most recent call last):
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/grpc_helpers.py", line 65, in error_remapped_callable
return callable_(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/grpc/_channel.py", line 1181, in __call__
return _end_unary_response_blocking(state, call, False, None)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/grpc/_channel.py", line 1006, in _end_unary_response_blocking
raise _InactiveRpcError(state) # pytype: disable=not-instantiable
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
status = StatusCode.UNKNOWN
details = ""
debug_error_string = "UNKNOWN:Error received from peer {created_time:"2025-03-07T15:46:47.764377+01:00", grpc_status:2, grpc_message:""}"
>

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/Users/artoale/dev/gcloudc/example.py", line 25, in
main()
File "/Users/artoale/dev/gcloudc/example.py", line 21, in main
client.reserve_ids_sequential(key, 1)
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/cloud/datastore/client.py", line 976, in reserve_ids_sequential
self._datastore_api.reserve_ids(
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/cloud/datastore_v1/services/datastore/client.py", line 1352, in reserve_ids
response = rpc(
^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/gapic_v1/method.py", line 113, in __call__
return wrapped_func(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/retry.py", line 349, in retry_wrapped_func
return retry_target(
^^^^^^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/retry.py", line 191, in retry_target
return target()
^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/timeout.py", line 120, in func_with_timeout
return func(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^
File "/Users/artoale/.pyenv/versions/3.11.9/lib/python3.11/site-packages/google/api_core/grpc_helpers.py", line 67, in error_remapped_callable
raise exceptions.from_grpc_error(exc) from exc
google.api_core.exceptions.Unknown: None
```

#### Emulator logs

```
[firestore] WARNING: Operation failed: Cannot invoke "com.google.cloud.datastore.core.rep.IdAllocationPolicy.equals(Object)" because "idPolicy" is null
[firestore] java.lang.NullPointerException: Cannot invoke "com.google.cloud.datastore.core.rep.IdAllocationPolicy.equals(Object)" because "idPolicy" is null
[firestore] at com.google.cloud.datastore.emulator.impl.firestore.FirestoreEmulatorHelper.filterIdsByScatteredPolicy(FirestoreEmulatorHelper.java:471)
[firestore] at com.google.cloud.datastore.emulator.impl.firestore.FirestoreEmulatorHelper.reserveIds(FirestoreEmulatorHelper.java:454)
[firestore] at com.google.cloud.datastore.emulator.impl.firestore.cloudv1.CloudDatastoreV1.reserveIds(CloudDatastoreV1.java:560)
[firestore] at com.google.cloud.datastore.emulator.impl.firestore.cloudv1.CloudDatastoreV1Router.reserveIds(CloudDatastoreV1Router.java:132)
[firestore] at com.google.cloud.datastore.emulator.firestore.cloudv1.CloudDatastoreV1GrpcAdapter$1.lambda$reserveIds$6(CloudDatastoreV1GrpcAdapter.java:98)
[firestore] at com.google.cloud.datastore.emulator.firestore.cloudv1.CloudDatastoreV1GrpcAdapter.unary(CloudDatastoreV1GrpcAdapter.java:49)
[firestore] at com.google.cloud.datastore.emulator.firestore.cloudv1.CloudDatastoreV1GrpcAdapter$1.reserveIds(CloudDatastoreV1GrpcAdapter.java:98)
[firestore] at com.google.datastore.v1.DatastoreGrpc$MethodHandlers.invoke(DatastoreGrpc.java:719)
[firestore] at io.grpc.stub.ServerCalls$UnaryServerCallHandler$UnaryServerCallListener.onHalfClose(ServerCalls.java:182)
[firestore] at io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.halfClosed(ServerCallImpl.java:356)
[firestore] at io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1HalfClosed.runInContext(ServerImpl.java:861)
[firestore] at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
[firestore] at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133)
[firestore] at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
[firestore] at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
[firestore] at java.base/java.lang.Thread.run(Thread.java:1575)
[firestore]
```

I've tested the same script against a real firestore in datastore mode entity and it works