fix(storage): MemoryStore.list_prefix uses directory-boundary semantics

[NIK-TIGER-BILL]

Summary

Fixes #3773

MemoryStore.list_prefix("0") was returning keys from sibling paths (e.g. "0_c/zarr.json") because it used a raw str.startswith check. LocalStore correctly uses filesystem directory semantics and only returns items under the "0/" directory.

This PR makes MemoryStore.list_prefix consistent with LocalStore by normalising the prefix to always end with / before matching.

Root Cause

# Before (buggy)
if key.startswith(prefix):  # prefix="0" matches "0_c/zarr.json" ❌

# After (fixed)
prefix = prefix.rstrip("/") + "/"  # prefix="0/"
if key.startswith(prefix):  # "0/" does NOT match "0_c/zarr.json" ✅

Repro

import asyncio, zarr

async def list_prefix(store, prefix):
    return sorted([k async for k in store.list_prefix(prefix)])

def run(store):
    root = zarr.open_group(store, mode="w")
    root.create_group("0")
    root.create_array("0_c", shape=(1,), dtype="i4")
    print(asyncio.run(list_prefix(store, "0")))

print("Before fix - MemoryStore:")  # [0/zarr.json, 0_c/zarr.json] ❌
run(zarr.storage.MemoryStore())
print("After fix - MemoryStore:")   # [0/zarr.json] ✅

Changes

• src/zarr/storage/_memory.py: list_prefix now normalises the prefix to end with /
• tests/test_store/test_memory.py: regression test test_list_prefix_path_boundary

Closes #3773