fix: isolate sandbox reads from request transactions

[wgu9]

Summary

Inject the existing async_sessionmaker into RemoteSandboxService and use short-lived sessions for pure database reads that are followed by runtime API calls. This keeps those reads from opening or committing the request-scoped transaction.

The isolated path now covers get_sandbox, search_sandboxes, get_sandbox_by_session_api_key, batch_get_sandboxes, and the database cross-reference in _get_user_running_sandboxes. Write paths continue using the shared request session, so their existing atomic behavior is unchanged.

This implements the productionization direction from OpenHands/OpenHands#16374 without relying on SQLAlchemy private APIs.

Closes OpenHands/OpenHands#16374

Testing

• TMPDIR=/tmp .venv/bin/pytest -q tests/unit/app_server: 1501 passed
• tests/unit/app_server/test_remote_sandbox_service.py: 112 passed
• full pre-commit: trailing whitespace, EOF, YAML, debug statements, AppMode check, pyproject validation, Ruff, Ruff format, and mypy passed
• five real SQLite regression cases verify that pure read paths do not commit a pending sibling write on the request session