Improve PHP 8.1+ support by avoiding deprecation when SSH client dies

[clue]

This changeset improves PHP 8.1+ support by avoiding a deprecation notice when the SSH client process dies. When the ssh process is killed by a signal, for example when cancelling a pending connection attempt, its exit code is reported as null, and passing this as the exception code is deprecated as of PHP 8.1 (2021). This is really only a leftover that wasn't spotted in #32, so the new tests make sure both connectors no longer raise this deprecation looking forward.

Builds on top of #32, #48, #8, #11 and #47

[Copilot]

🟢 Approval recommended

The narrow fix handles null exit codes while preserving integer codes, with regression coverage and no identified blocking issues.

No open issues 🎉

What changed in this PR

Prevents PHP 8.1+ deprecation notices when an SSH process exits with a null exit code.

Changes:

• Casts exit codes to integers in both connectors.
• Adds regression tests for connection cancellation and idle-process termination.

File	Description
tests/​IntegrationSshSocksConnectorTest.php	Tests termination after a SOCKS connection failure.
tests/​IntegrationSshProcessConnectorTest.php	Tests pending connection cancellation.
src/​SshSocksConnector.php	Casts the exception code to an integer.
src/​SshProcessConnector.php	Casts the exception code to an integer.

_{Review effort: Balanced}