Transitioned activity bounded context to slog. (#39773)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #38889 

# Checklist for submitter

- [ ] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
  - Changes file already exists

## Testing

- [x] Added/updated automated tests
- [x] QA'd all new/changed functionality manually


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated internal logging infrastructure across activity service
modules to enhance code consistency and maintainability.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

server/activity/internal/testutils/testutils.go

@@ -3,12 +3,14 @@ package testutils
 
 import (
 	"encoding/json"
+	"log/slog"
 	"testing"
 	"time"
 
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	common_mysql "github.com/fleetdm/fleet/v4/server/platform/mysql"
 	mysql_testing_utils "github.com/fleetdm/fleet/v4/server/platform/mysql/testing_utils"
-	"github.com/go-kit/log"
+	kitlog "github.com/go-kit/log"
 	"github.com/jmoiron/sqlx"
 	"github.com/stretchr/testify/require"
 )
@@ -16,7 +18,7 @@ import (
 // TestDB holds the database connection for tests.
 type TestDB struct {
 	DB     *sqlx.DB
-	Logger log.Logger
+	Logger *slog.Logger
 }
 
 // SetupTestDB creates a test database with the Fleet schema loaded.
@@ -36,7 +38,7 @@ func SetupTestDB(t *testing.T, testNamePrefix string) *TestDB {
 
 	return &TestDB{
 		DB:     db,
-		Logger: log.NewNopLogger(),
+		Logger: slog.New(logging.DiscardHandler{}),
 	}
 }
 
@@ -48,7 +50,7 @@ func (tdb *TestDB) Conns() *common_mysql.DBConnections {
 // TruncateTables clears the tables used by activity bounded context.
 func (tdb *TestDB) TruncateTables(t *testing.T) {
 	t.Helper()
-	mysql_testing_utils.TruncateTables(t, tdb.DB, tdb.Logger, nil, "host_activities", "activities", "hosts", "users")
+	mysql_testing_utils.TruncateTables(t, tdb.DB, kitlog.NewNopLogger(), nil, "host_activities", "activities", "hosts", "users")
 }
 
 // InsertUser creates a user in the database and returns the user ID.