test(python): cover concurrent S3 table opens (#3833)

## Summary

- add regression coverage for the reported synchronous Python workload
with 32 simultaneous `open_table` calls
- verify every independently opened S3-backed table handle can read
through the connection's shared session and object-store client

## Root cause

In Python v0.13.0, each synchronous table handle lazily constructed its
own Lance dataset. Opening many handles in parallel therefore triggered
independent S3 client construction and bucket-region resolution, which
failed under thread pressure. The current Rust-backed connection path
owns a shared Lance session and retains its object-store handle, so
table opens reuse the existing S3 client; these tests lock in that
behavior through the public Python API and a causal Session-registry
invariant.

## Validation

- `uvx --from 'ruff==0.15.20' ruff format --check
python/tests/test_s3.py`
- `uvx --from 'ruff==0.15.20' ruff check .`
- `cargo fmt --all`
- `cargo test --quiet --features remote -p lancedb
test_concurrent_open_table_reuses_connection_object_store`
- `cargo check --quiet --features remote --tests --examples`
- equivalent 32-thread `open_table(...).count_rows()` workload against a
local database
- targeted S3 test collected successfully locally; execution requires
the CI LocalStack service, which is unavailable in this runner

Fixes #1786

<!-- lance-gatekeeper-fix:v1 agent=d311f3c7151f77ae22b4997702e7b7db
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
Co-authored-by: Xuanwo <github@xuanwo.io>

python/python/tests/test_s3.py

@@ -4,6 +4,7 @@
 
 import asyncio
 import copy
+from concurrent.futures import ThreadPoolExecutor
 from datetime import timedelta
 import threading
 
@@ -86,6 +87,25 @@ def test_s3_lifecycle(s3_bucket: str):
     asyncio.run(test())
 
 
+@pytest.mark.s3_test
+def test_concurrent_open_table(s3_bucket: str):
+    uri = f"s3://{s3_bucket}/test_concurrent_open_table"
+    db = lancedb.connect(uri, storage_options=copy.copy(CONFIG))
+    db.create_table("test", pa.table({"x": [1, 2, 3]}))
+
+    num_workers = 32
+    barrier = threading.Barrier(num_workers)
+
+    def open_and_count(_):
+        barrier.wait()
+        return db.open_table("test").count_rows()
+
+    with ThreadPoolExecutor(max_workers=num_workers) as pool:
+        row_counts = list(pool.map(open_and_count, range(num_workers)))
+
+    assert row_counts == [3] * num_workers
+
+
 @pytest.fixture()
 def kms_key():
     kms = get_boto3_client("kms", endpoint_url=CONFIG["aws_endpoint"])