feat: improve agent management and speed up tests

backend/src/Monitor.Backend.Api/Contracts/AdminDtos.cs

@@ -46,7 +46,9 @@ public sealed record JoinTokenDto(string Id, string? Note, IReadOnlyList<string>
 public sealed record CreateJoinTokenRequest(string? Note, IReadOnlyList<string>? Labels, bool? AutoApprove, int? MaxUses, int? ExpiresInHours);
 
 /// <summary>令牌明文只在创建的响应里出现一次。</summary>
-public sealed record CreatedJoinTokenDto(JoinTokenDto Token, string Plaintext, string InstallCommand);
+public sealed record InstallCommandsDto(string Linux, string Macos, string Windows);
+
+public sealed record CreatedJoinTokenDto(JoinTokenDto Token, string Plaintext, InstallCommandsDto InstallCommands);
 
 /// <summary>Agent 眼中连着的一个 Probe（Agent 上报）。</summary>
 public sealed record ConnectedProbeDto(string ProbeId, string? Version, string? Remote, DateTimeOffset ConnectedSince);