feat: full end to end testing

[fmartingr]

Add testing infrastructure and user synchronization:

• E2E and integration test framework with testhelper package

  func TestSlackImportE2E(t *testing.T) {
       // Setup: Automatically creates Docker containers (network, PostgreSQL, Mattermost)
       th := testhelper.SetupHelper(t)
       defer th.TearDown()
       
       // th.Client is authenticated API client
       // th.SiteURL is the running Mattermost URL
       // th.AdminUser is the admin account
       
       // Create a team
       team, _, err := th.Client.CreateTeam(&model.Team{
           Name:        "myteam",
           DisplayName: "My Team",
           Type:        model.TeamOpen,
       })
       require.NoError(t, err)
       
       // Import data and verify it worked
       err = th.ImportJSONL("path/to/import.jsonl")
       require.NoError(t, err)
       
       // Validate imported users exist
       validation := th.ValidateImportedUsers([]string{"john.doe", "jane.smith"})
       assert.True(t, validation.Valid)
   }

• Slack export fixtures and validation utilities

// Build a Slack export programmatically using fluent API
err := slack.NewSlackExportBuilder().
    AddUser(slack.SlackUser{
        Id:       "U001",
        Username: "john.doe",
        Profile: slack.SlackProfile{
            RealName: "John Doe",
            Email:    "john@example.com",
            Title:    "Engineer",
        },
    }).
    AddUser(slack.SlackUser{
        Id:       "U002",
        Username: "jane.smith",
        Profile: slack.SlackProfile{
            Email: "jane@example.com",
        },
    }).
    AddChannel(slack.SlackChannel{
        Id:      "C001",
        Name:    "general",
        Creator: "U001",
        Members: []string{"U001", "U002"},
        Purpose: slack.SlackChannelSub{Value: "General discussion"},
        Topic:   slack.SlackChannelSub{Value: "Welcome!"},
    }).
    AddPost("general", slack.SlackPost{
        User:      "U001",
        Text:      "Hello world!",
        Timestamp: "1577836800.000100",
        Type:      "message",
    }).
    AddPost("general", slack.SlackPost{
        User:      "U002",
        Text:      "Hi everyone!",
        Timestamp: "1577836801.000200",
        Type:      "message",
    }).
    Build(exportPath)