Manages browser sessions for different threads.
This class maintains separate browser sessions for different threads,
enabling concurrent usage of browsers in multi-threaded environments.
Browsers are created lazily only when needed by tools.
Concurrency protection is also implemented. Each browser session is tied
to a specific thread_id and includes protection against concurrent usage.
When a browser is obtained via get_async_browser() or get_sync_browser(),
it is marked as "in use", and subsequent attempts to access the same
browser session will raise a RuntimeError until it is released. In general,
different callers should use different thread_ids to avoid concurrency issues.
