Changes needed before gokit/log to slog transition. (#39527)

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

PLEASE READ BELOW before looking at file changes

Before converting individual files/packages to slog, we generally need
to make these 2 changes to make the conversion easier:
- Replace uses of `kitlog.With` since they are not fully compatible with
our kitlog adapter
- Directly use the kitlog adapter logger type instead of the kitlog
interface, which will let us have direct access to the underlying slog
logger: `*logging.Logger`

Note: that I did not replace absolutely all uses of `kitlog.Logger`, but
I did remove all uses of `kitlog.With` except for these due to
complexity:
- server/logging/filesystem.go and the other log writers (webhook,
firehose, kinesis, lambda, pubsub, nats)
- server/datastore/mysql/nanomdm_storage.go (adapter pattern)
- server/vulnerabilities/nvd/* (cascades to CLI tools)
- server/service/osquery_utils/queries.go (callback type signatures
cascade broadly)
- cmd/maintained-apps/ (standalone, so can be transitioned later all at
once)

Most of the changes in this PR follow these patterns:
- `kitlog.Logger` type → `*logging.Logger`
- `kitlog.With(logger, ...)` → `logger.With(...)`
- `kitlog.NewNopLogger() → logging.NewNopLogger()`, including similar
variations such as `logging.NewLogfmtLogger(w)` and
`logging.NewJSONLogger(w)`
- removed many now-unused kitlog imports

Unique changes that the PR review should focus on:
- server/platform/logging/kitlog_adapter.go: Core adapter changes
- server/platform/logging/logging.go: New convenience functions
- server/service/integration_logger_test.go: Test changes for slog

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
  - Was added in previous PR

## 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

* **Refactor**
* Migrated the codebase to a unified internal structured logging system
for more consistent, reliable logs and observability.
* No user-facing functionality changed; runtime behavior and APIs remain
compatible.
* **Tests**
* Updated tests to use the new logging helpers to ensure consistent test
logging and validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

ee/server/calendar/google_calendar.go

@@ -15,7 +15,7 @@ import (
 	"github.com/cenkalti/backoff/v4"
 	"github.com/fleetdm/fleet/v4/server/contexts/ctxerr"
 	"github.com/fleetdm/fleet/v4/server/fleet"
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/go-kit/log/level"
 	"github.com/google/uuid"
 	"golang.org/x/oauth2/google"
@@ -53,7 +53,7 @@ var (
 type GoogleCalendarConfig struct {
 	Context           context.Context
 	IntegrationConfig *fleet.GoogleCalendarIntegration
-	Logger            kitlog.Logger
+	Logger            *logging.Logger
 	ServerURL         string
 	// Should be nil for production
 	API GoogleCalendarAPI
@@ -108,7 +108,7 @@ type eventDetails struct {
 
 type GoogleCalendarLowLevelAPI struct {
 	service   *calendar.Service
-	logger    kitlog.Logger
+	logger    *logging.Logger
 	serverURL string
 }
 

ee/server/calendar/google_calendar_integration_test.go

@@ -9,7 +9,7 @@ import (
 
 	"github.com/fleetdm/fleet/v4/ee/server/calendar/load_test"
 	"github.com/fleetdm/fleet/v4/server/fleet"
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -62,7 +62,7 @@ func (s *googleCalendarIntegrationTestSuite) TestCreateGetDeleteEvent() {
 				"private_key":  s.server.URL,
 			}},
 		},
-		Logger: kitlog.NewLogfmtLogger(kitlog.NewSyncWriter(os.Stdout)),
+		Logger: logging.NewLogfmtLogger(os.Stdout),
 	}
 	gCal := NewGoogleCalendar(config)
 	err := gCal.Configure(userEmail)
@@ -129,7 +129,7 @@ func (s *googleCalendarIntegrationTestSuite) TestFillUpCalendar() {
 				"private_key":  s.server.URL,
 			}},
 		},
-		Logger: kitlog.NewLogfmtLogger(kitlog.NewSyncWriter(os.Stdout)),
+		Logger: logging.NewLogfmtLogger(os.Stdout),
 	}
 	gCal := NewGoogleCalendar(config)
 	err := gCal.Configure(userEmail)

ee/server/calendar/google_calendar_load.go

@@ -7,7 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"github.com/fleetdm/fleet/v4/pkg/fleethttp"
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"google.golang.org/api/calendar/v3"
 	"google.golang.org/api/googleapi"
 	"io"
@@ -18,7 +18,7 @@ import (
 
 // GoogleCalendarLoadAPI is used for load testing.
 type GoogleCalendarLoadAPI struct {
-	Logger            kitlog.Logger
+	Logger            *logging.Logger
 	baseUrl           string
 	userToImpersonate string
 	ctx               context.Context
@@ -30,7 +30,7 @@ type GoogleCalendarLoadAPI struct {
 func (lowLevelAPI *GoogleCalendarLoadAPI) Configure(ctx context.Context, _ string, privateKey string, userToImpersonate string,
 	serverURL string) error {
 	if lowLevelAPI.Logger == nil {
-		lowLevelAPI.Logger = kitlog.With(kitlog.NewLogfmtLogger(os.Stderr), "mock", "GoogleCalendarLoadAPI", "user", userToImpersonate)
+		lowLevelAPI.Logger = logging.NewLogfmtLogger(os.Stderr).With("mock", "GoogleCalendarLoadAPI", "user", userToImpersonate)
 	}
 	lowLevelAPI.baseUrl = privateKey
 	lowLevelAPI.userToImpersonate = userToImpersonate

ee/server/calendar/google_calendar_mock.go

@@ -10,13 +10,13 @@ import (
 	"sync"
 	"time"
 
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"google.golang.org/api/calendar/v3"
 	"google.golang.org/api/googleapi"
 )
 
 type GoogleCalendarMockAPI struct {
-	logger kitlog.Logger
+	logger *logging.Logger
 }
 
 type channel struct {
@@ -36,7 +36,7 @@ const latency = 200 * time.Millisecond
 // Configure creates a new Google Calendar service using the provided credentials.
 func (lowLevelAPI *GoogleCalendarMockAPI) Configure(_ context.Context, _ string, _ string, userToImpersonate string, _ string) error {
 	if lowLevelAPI.logger == nil {
-		lowLevelAPI.logger = kitlog.With(kitlog.NewLogfmtLogger(os.Stderr), "mock", "GoogleCalendarMockAPI", "user", userToImpersonate)
+		lowLevelAPI.logger = logging.NewLogfmtLogger(os.Stderr).With("mock", "GoogleCalendarMockAPI", "user", userToImpersonate)
 	}
 	return nil
 }

ee/server/calendar/google_calendar_test.go

@@ -10,7 +10,7 @@ import (
 	"time"
 
 	"github.com/fleetdm/fleet/v4/server/fleet"
-	"github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/api/calendar/v3"
@@ -26,7 +26,7 @@ const (
 
 var (
 	baseCtx = context.Background()
-	logger  = log.NewLogfmtLogger(log.NewSyncWriter(os.Stdout))
+	logger  = logging.NewLogfmtLogger(os.Stdout)
 )
 
 type MockGoogleCalendarLowLevelAPI struct {