`open_reader` double-applies a store `prefix=`

[blinkybool]

On a store created with a prefix= (e.g. S3Store(prefix="P")), obstore.open_reader(store, "key") resolves the object at P/P/key instead of P/key, so every buffered read of a prefixed store fails with a 404. get, put, and list on the same store are correct (single prefix). Regressed in 0.9.0; still present in 0.11.0.

Environment

• obstore 0.11.0 also reproduced on 0.9.0.
• Last good: 0.8.2

Reproduction

Standalone uv run repro.py (deps declared inline; moto provides a real S3 backend). The object is written once at P/key; swap the obstore pin to 0.8.2 to see the correct behavior.

# /// script
# requires-python = ">=3.11"
# dependencies = ["obstore==0.11.0", "boto3==1.43.22", "moto[server]==5.2.1"]
# ///
import boto3, obstore
from moto.server import ThreadedMotoServer
from obstore.store import S3Store

server = ThreadedMotoServer(port=0); server.start()
host, port = server.get_host_and_port()
endpoint = f"http://{host}:{port}"

s3 = boto3.client("s3", endpoint_url=endpoint, region_name="us-east-1",
                  aws_access_key_id="x", aws_secret_access_key="x")
s3.create_bucket(Bucket="bucket")
s3.put_object(Bucket="bucket", Key="P/key", Body=b"hello")  # object lives at P/key

store = S3Store(bucket="bucket", prefix="P", endpoint=endpoint, region="us-east-1",
                access_key_id="x", secret_access_key="x",
                client_options={"allow_http": True}, virtual_hosted_style_request=False)

print("list :", [o["path"] for b in obstore.list(store) for o in b])  # ['key']
print("head :", obstore.head(store, "key")["path"])                    # 'P/key'  <- should be 'key'
print("get  :", bytes(obstore.get(store, "key").bytes()))              # b'hello'
try:
    print("read :", obstore.open_reader(store, "key").read())          # -> GET P/P/key -> 404
except Exception as e:
    print("read :", str(e).splitlines()[0])
server.stop()

Output

obstore 0.11.0 (bug) — moto also logs HEAD /bucket/P/key then GET /bucket/P/P/key:

list : ['key']       # prefix stripped (correct)
head : P/key         # <- prefix NOT stripped (root cause)
get  : b'hello'      # correct
read : Object at location P/P/key not found: ... GET .../bucket/P/P/key ... 404

obstore 0.8.2 (correct) — GET goes to /bucket/P/key:

list : ['key']
head : key
get  : b'hello'
read : Bytes(b'hello')

Expected

open_reader(store, "key") on a store with prefix="P" should read P/key (as get/put/list do), and head(...)["path"] should return "key" (prefix-relative), consistent with list().