Fix race conditions and stale SSH client process in SshSocksConnector

[clue]

This changeset fixes several race conditions and a stale SSH client process in SshSocksConnector. These date back to the shared SSH client process from #8 and only show when a connection attempt is cancelled or the SSH client exits at the wrong moment:

• Cancelling while the SSH client fails would crash with Call to a member function then() on null.
• An SSH client exiting late would clear the reference to a newer one, which then kept running.
• Cancelling before the SSH client was ready would still open a SOCKS connection and leak it.
• An SSH client dying after it was ready went unnoticed, so connection attempts kept failing until all connections were closed.

Each fix comes in its own commit with a test reproducing the original problem.

Builds on top of #52, #50 and #8
Refs clue/reactphp-redis#105 and friends-of-reactphp/mysql#110

[Copilot]

🟡 Changes recommended

The stale-process regression test must distinguish connector-triggered termination from natural process exit.

1 open issue

• Test does not verify SIGTERM-triggered process termination · New

What changed in this PR

Fixes shared SSH process lifecycle races in SshSocksConnector.

Changes:

• Prevents cancelled attempts from opening SOCKS connections.
• Protects replacement process references and clears exited processes.
• Adds regression tests for cancellation and process-exit timing.

File	Description
tests/​IntegrationSshSocksConnectorTest.php	Adds four lifecycle regression tests.
src/​SshSocksConnector.php	Corrects cancellation, idle cleanup, and process-exit handling.

_{Review effort: Balanced}