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/service/condaccess/idp.go

@@ -20,6 +20,7 @@ import (
 	"github.com/fleetdm/fleet/v4/server/contexts/ctxerr"
 	"github.com/fleetdm/fleet/v4/server/dev_mode"
 	"github.com/fleetdm/fleet/v4/server/fleet"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/fleetdm/fleet/v4/server/service/middleware/log"
 	"github.com/fleetdm/fleet/v4/server/service/middleware/otel"
 	kitlog "github.com/go-kit/log"
@@ -61,7 +62,7 @@ func (e *notFoundError) IsNotFound() bool {
 // idpService implements the Okta conditional access IdP functionality.
 type idpService struct {
 	ds               fleet.Datastore
-	logger           kitlog.Logger
+	logger           *logging.Logger
 	certSerialFormat string
 }
 
@@ -69,7 +70,7 @@ type idpService struct {
 func RegisterIdP(
 	mux *http.ServeMux,
 	ds fleet.Datastore,
-	logger kitlog.Logger,
+	logger *logging.Logger,
 	fleetConfig *config.FleetConfig,
 ) error {
 	if fleetConfig == nil {
@@ -78,7 +79,7 @@ func RegisterIdP(
 
 	svc := &idpService{
 		ds:               ds,
-		logger:           kitlog.With(logger, "component", "conditional-access-idp"),
+		logger:           logger.With("component", "conditional-access-idp"),
 		certSerialFormat: fleetConfig.ConditionalAccess.CertSerialFormat,
 	}
 
@@ -565,7 +566,7 @@ func (s *idpService) buildIdentityProvider(ctx context.Context, serverURL string
 	ssoURL = ssoURL.JoinPath(idpSSOPath)
 
 	// Create kitlog adapter for SAML library
-	samlLogger := &kitlogAdapter{logger: kitlog.With(s.logger, "component", "saml-idp")}
+	samlLogger := &kitlogAdapter{logger: s.logger.With("component", "saml-idp")}
 
 	// Build IdentityProvider
 	// Note: SessionProvider is set dynamically in serveSSO based on the authenticated device

ee/server/service/condaccess/idp_test.go

@@ -15,8 +15,8 @@ import (
 	"github.com/fleetdm/fleet/v4/server/config"
 	"github.com/fleetdm/fleet/v4/server/fleet"
 	"github.com/fleetdm/fleet/v4/server/mock"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	common_mysql "github.com/fleetdm/fleet/v4/server/platform/mysql"
-	kitlog "github.com/go-kit/log"
 	"github.com/jmoiron/sqlx"
 	"github.com/stretchr/testify/require"
 )
@@ -76,13 +76,13 @@ b1ctZeF7HaWwFdTC8GqWI6zzRFn+YA3f/yYibhowuEypPQeSjlI=
 
 func newTestService() (*idpService, *mock.Store) {
 	ds := new(mock.Store)
-	logger := kitlog.NewNopLogger()
+	logger := logging.NewNopLogger()
 	return &idpService{ds: ds, logger: logger, certSerialFormat: config.CertSerialFormatHex}, ds
 }
 
 func newTestServiceWithCertFormat(certFormat string) (*idpService, *mock.Store) {
 	ds := new(mock.Store)
-	logger := kitlog.NewNopLogger()
+	logger := logging.NewNopLogger()
 	return &idpService{ds: ds, logger: logger, certSerialFormat: certFormat}, ds
 }
 
@@ -117,7 +117,7 @@ func mockCertAssetsFunc(includeCerts bool) func(context.Context, []fleet.MDMAsse
 
 func TestRegisterIdP(t *testing.T) {
 	ds := new(mock.Store)
-	logger := kitlog.NewNopLogger()
+	logger := logging.NewNopLogger()
 	cfg := &config.FleetConfig{}
 
 	ds.AppConfigFunc = mockAppConfigFunc("https://fleet.example.com")
@@ -526,7 +526,7 @@ func TestParseCertAndKeyBytes(t *testing.T) {
 }
 
 func TestDeviceHealthSessionProvider(t *testing.T) {
-	logger := kitlog.NewNopLogger()
+	logger := logging.NewNopLogger()
 	now := time.Now()
 
 	tests := []struct {

ee/server/service/condaccess/log_adapter.go

@@ -3,13 +3,13 @@ package condaccess
 import (
 	"fmt"
 
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/go-kit/log/level"
 )
 
-// kitlogAdapter adapts kitlog.Logger to saml logger.Interface
+// kitlogAdapter adapts to saml logger.Interface
 type kitlogAdapter struct {
-	logger kitlog.Logger
+	logger *logging.Logger
 }
 
 func (k *kitlogAdapter) Printf(format string, v ...interface{}) {

ee/server/service/condaccess/scep.go

@@ -15,9 +15,9 @@ import (
 	"github.com/fleetdm/fleet/v4/server/mdm/assets"
 	scepdepot "github.com/fleetdm/fleet/v4/server/mdm/scep/depot"
 	scepserver "github.com/fleetdm/fleet/v4/server/mdm/scep/server"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/fleetdm/fleet/v4/server/service/middleware/otel"
 	"github.com/go-kit/kit/log"
-	kitlog "github.com/go-kit/log"
 	"github.com/smallstep/scep"
 )
 
@@ -45,7 +45,7 @@ func RegisterSCEP(
 	mux *http.ServeMux,
 	scepStorage scepdepot.Depot,
 	ds fleet.Datastore,
-	logger kitlog.Logger,
+	logger *logging.Logger,
 	fleetConfig *config.FleetConfig,
 ) error {
 	if fleetConfig == nil {
@@ -68,10 +68,10 @@ func RegisterSCEP(
 	scepService := NewSCEPService(
 		ds,
 		signer,
-		kitlog.With(logger, "component", "conditional-access-scep"),
+		logger.With("component", "conditional-access-scep"),
 	)
 
-	scepLogger := kitlog.With(logger, "component", "http-conditional-access-scep")
+	scepLogger := logger.With("component", "http-conditional-access-scep")
 	e := scepserver.MakeServerEndpoints(scepService)
 	e.GetEndpoint = scepserver.EndpointLoggingMiddleware(scepLogger)(e.GetEndpoint)
 	e.PostEndpoint = scepserver.EndpointLoggingMiddleware(scepLogger)(e.PostEndpoint)