fix(browser): register event loop on current thread in _execute_async

[xlyoung]

Summary

Fixes #453

When stream_async() dispatches _execute_async to a worker thread, the event loop created in Browser.__init__() is not registered on that thread. This causes self._loop.run_until_complete() to fail with:

RuntimeError: There is no current event loop in thread 'asyncio_0'

Root Cause

• __init__ calls asyncio.set_event_loop(self._loop) on the creating thread
• _execute_async is a synchronous method that Strands dispatches to a different thread via _stream
• The worker thread has no event loop registered, so run_until_complete raises

Fix

Add asyncio.set_event_loop(self._loop) at the start of _execute_async to ensure the loop is registered on whichever thread executes the method. This is idempotent and safe - if the loop is already set, it's a no-op.

Testing

• Verified that AgentCoreBrowser works with agent.stream_async() when called from within an already-running async event loop
• Existing sync agent() calls continue to work unchanged

Closes #453