fix(libev): guard handle_read/handle_write against close() race condition

[vponomaryov]

When close() is called from one thread, it sets is_closed=True and closes the socket immediately. However, libev watchers are stopped asynchronously in _loop_will_run(), so handle_read()/handle_write() can still fire on the now-closed fd, causing EBADF errors that surface as ConnectionShutdown('Bad file descriptor') and prevent reconnection.

So, fix it applying following changes:

• Early-return guards at the top of handle_read() and handle_write() that check is_closed/is_defunct before touching the socket
• Secondary is_closed/is_defunct checks in error handlers to catch the race when close() happens between watcher dispatch and syscall
• Peer disconnect detection (EBADF, ECONNRESET, ENOTCONN, etc.) that calls close() cleanly instead of defunct()
• last_error preservation in close() when connected_event is unset, preventing factory() from returning dead connections

Fixes: #614

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[vponomaryov]

Ref: https://scylladb.atlassian.net/browse/SCT-83