fix(remote): validate cloud DNS hostnames (#3845)

## Summary

- validate the generated LanceDB Cloud hostname during connection setup
- return a clear invalid-input error for empty, overlong, or oversized
DNS names before network resolution
- add Rust and Python regression coverage for malformed `db://`
authorities

## Root cause

The `db://` authority and region were interpolated into the Cloud API
hostname without DNS length validation. Empty or overlong labels
therefore reached the resolver and surfaced as an opaque IDNA
`UnicodeError` instead of a useful connection error.

## Validation

- `cargo test --quiet --features remote -p lancedb
test_rejects_invalid_cloud_dns_hostname --lib`
- `cargo check --quiet --features remote --tests --examples`
- `uv run --no-sync --extra tests pytest
python/tests/test_remote_db.py::test_async_remote_db
python/tests/test_remote_db.py::test_connect_rejects_invalid_cloud_dns_hostname
-q`
- `cargo fmt --all -- --check`
- `ruff check .`
- `ruff format --check python/python/tests/test_remote_db.py`

Fixes #799

<!-- lance-gatekeeper-fix:v1 agent=4d1597b3d244b58f0603ed40a8a59cf9
generation=1 -->

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

python/python/tests/test_remote_db.py

@@ -35,6 +35,12 @@ def make_mock_http_handler(handler):
     return MockLanceDBHandler
 
 
+@pytest.mark.parametrize("db_name", ["a" * 64, "invalid..database"])
+def test_connect_rejects_invalid_cloud_dns_hostname(db_name):
+    with pytest.raises(ValueError, match="DNS labels must contain 1 to 63 bytes"):
+        lancedb.connect(f"db://{db_name}", api_key="fake")
+
+
 @contextlib.contextmanager
 def mock_lancedb_connection(handler):
     with http.server.HTTPServer(