feat(ssh): add FleetService::testConnection credential probe

Connect + trivial exec against a server's stored credential; never throws,
returns {ok,error}. Used by the create-server guard to verify the SSH login
before persisting.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/Services/FleetService.php

@@ -118,6 +118,29 @@ class FleetService
         return array_map(fn (array $l): string => implode("\n", $l), $parts);
     }
 
+    /**
+     * Probe the server's stored credential: connect + a trivial exec. NEVER throws —
+     * returns the failure reason instead, for the create-server guard to surface.
+     * Uses a short timeout so an unreachable host fails fast.
+     *
+     * @return array{ok: bool, error: ?string}
+     */
+    public function testConnection(Server $server): array
+    {
+        try {
+            $ssh = (new SshClient($this->vault, timeout: 10))->connect($server);
+            try {
+                [, $code] = $ssh->run('true');
+            } finally {
+                $ssh->disconnect();
+            }
+
+            return ['ok' => $code === 0, 'error' => null];
+        } catch (Throwable $e) {
+            return ['ok' => false, 'error' => $e->getMessage()];
+        }
+    }
+
     // ── Metrics (light — used by the poller and dashboard fallback) ──────
 
     /** @return array{cpu:int,mem:int,disk:int,load:float} */

tests/Feature/FleetTestConnectionTest.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Tests\Feature;
+
+use App\Models\Server;
+use App\Services\FleetService;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+class FleetTestConnectionTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_unreachable_host_returns_error_without_throwing(): void
+    {
+        $server = Server::create(['name' => 'x', 'ip' => '127.0.0.1', 'ssh_port' => 1, 'status' => 'pending']);
+        $server->credential()->create([
+            'username' => 'root', 'auth_type' => 'password', 'secret' => 'nope',
+        ]);
+        $server->load('credential');
+
+        $res = app(FleetService::class)->testConnection($server);
+
+        $this->assertFalse($res['ok']);
+        $this->assertNotNull($res['error']);
+    }
+}