Fix mysql reconnection test: proxy must close both sides on planned disconnect

[Copilot]

pitr_binlog_server_reconnection_test.sh consistently failed with ~286/301 rows replicated because binlog_proxy.py was reconnecting only the server-side socket while keeping the MySQL client connection open.

Root cause

When the proxy triggered a planned disconnect, it closed and reopened its connection to the wal-g binlog server while holding the MySQL connection. The binlog server immediately issued a fresh MySQL protocol handshake on the new TCP connection; the proxy dutifully forwarded that handshake to MySQL—which was mid-stream expecting binlog events. MySQL's IO thread flagged a protocol violation, entered Connecting state, and never recovered through the same session.

Fix

On a planned disconnect, close both the client and server sockets and return from handle_client_connection. The outer start() accept loop then picks up MySQL's standard reconnect (governed by MASTER_CONNECT_RETRY/MASTER_RETRY_COUNT), which carries the correct GTID set and triggers a proper COM_BINLOG_DUMP_GTID against the binlog server's existing streamer.

# Before: only server socket closed, MySQL kept connected → protocol corruption
self.server_socket.close()
time.sleep(2)
if not self.connect_to_server():
    break
self.bytes_transferred = 0

# After: close both, return; outer loop accepts MySQL's clean reconnect
print(f"[Proxy] Closing both connections to force client reconnect")
return  # finally block closes both sockets

Because the 256-byte threshold fires during the TCP auth phase (before COM_BINLOG_DUMP_GTID), the two planned disconnects occur before replication actually starts. The third connection operates in stable mode and replicates all 301 rows cleanly.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] addressing mysql test failure</issue_title>
<issue_description>### What feature would you like?

we need to address the following failure https://github.com/wal-g/wal-g/actions/runs/22561524850/job/65349492528?pr=2187

Why is this feature useful?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE] addressing mysql test failure #48

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