Fix SSH multiplexing contention for concurrent scheduled tasks (#6736)

When multiple scheduled tasks or database backups run concurrently on
the same server, they compete for the same SSH multiplexed connection
socket, causing race conditions and SSH exit code 255 errors.

This fix adds a `disableMultiplexing` parameter to bypass SSH
multiplexing for jobs that may run concurrently:

- Add `disableMultiplexing` param to `generateSshCommand()`
- Add `disableMultiplexing` param to `instant_remote_process()`
- Update `ScheduledTaskJob` to use `disableMultiplexing: true`
- Update `DatabaseBackupJob` to use `disableMultiplexing: true`
- Add debug logging to track execution without multiplexing
- Add unit tests for the new parameter

Each backup and scheduled task now gets an isolated SSH connection,
preventing contention on the shared multiplexed socket.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Helpers/SshMultiplexingHelper.php

@@ -149,7 +149,7 @@ class SshMultiplexingHelper
         return $scp_command;
     }
 
-    public static function generateSshCommand(Server $server, string $command)
+    public static function generateSshCommand(Server $server, string $command, bool $disableMultiplexing = false)
     {
         if ($server->settings->force_disabled) {
             throw new \RuntimeException('Server is disabled.');
@@ -168,7 +168,7 @@ class SshMultiplexingHelper
         $ssh_command = "timeout $timeout ssh ";
 
         $multiplexingSuccessful = false;
-        if (self::isMultiplexingEnabled()) {
+        if (! $disableMultiplexing && self::isMultiplexingEnabled()) {
             try {
                 $multiplexingSuccessful = self::ensureMultiplexedConnection($server);
                 if ($multiplexingSuccessful) {