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 -->

server/service/schedule/schedule.go

@@ -14,7 +14,7 @@ import (
 
 	"github.com/fleetdm/fleet/v4/server/contexts/ctxerr"
 	"github.com/fleetdm/fleet/v4/server/fleet"
-	"github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/go-kit/log/level"
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
@@ -33,7 +33,7 @@ type Schedule struct {
 	ctx        context.Context
 	name       string
 	instanceID string
-	logger     log.Logger
+	logger     *logging.Logger
 
 	defaultPrevRunCreatedAt time.Time // default timestamp of previous run for the schedule if none exists, time.Now if not set
 
@@ -92,9 +92,9 @@ type CronStatsStore interface {
 type Option func(*Schedule)
 
 // WithLogger sets a logger for the Schedule.
-func WithLogger(l log.Logger) Option {
+func WithLogger(l *logging.Logger) Option {
 	return func(s *Schedule) {
-		s.logger = log.With(l, "schedule", s.name)
+		s.logger = l.With("schedule", s.name)
 	}
 }
 
@@ -168,7 +168,7 @@ func New(
 		ctx:                  ctx,
 		name:                 name,
 		instanceID:           instanceID,
-		logger:               log.NewNopLogger(),
+		logger:               logging.NewNopLogger(),
 		trigger:              make(chan struct{}),
 		done:                 make(chan struct{}),
 		configReloadInterval: 1 * time.Hour, // by default we will check for updated config once per hour
@@ -180,9 +180,9 @@ func New(
 		fn(sch)
 	}
 	if sch.logger == nil {
-		sch.logger = log.NewNopLogger()
+		sch.logger = logging.NewNopLogger()
 	}
-	sch.logger = log.With(sch.logger, "instanceID", instanceID)
+	sch.logger = sch.logger.With("instanceID", instanceID)
 	sch.errors = make(fleet.CronScheduleErrors)
 	return sch
 }