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/pubsub/redis_query_results.go

@@ -10,7 +10,7 @@ import (
 	"github.com/fleetdm/fleet/v4/server/contexts/ctxerr"
 	"github.com/fleetdm/fleet/v4/server/datastore/redis"
 	"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"
 	redigo "github.com/gomodule/redigo/redis"
 )
@@ -18,14 +18,14 @@ import (
 type redisQueryResults struct {
 	pool             fleet.RedisPool
 	duplicateResults bool
-	logger           log.Logger
+	logger           *logging.Logger
 }
 
 var _ fleet.QueryResultStore = &redisQueryResults{}
 
 // NewRedisQueryResults creats a new Redis implementation of the
 // QueryResultStore interface using the provided Redis connection pool.
-func NewRedisQueryResults(pool fleet.RedisPool, duplicateResults bool, logger log.Logger) *redisQueryResults {
+func NewRedisQueryResults(pool fleet.RedisPool, duplicateResults bool, logger *logging.Logger) *redisQueryResults {
 	return &redisQueryResults{
 		pool:             pool,
 		duplicateResults: duplicateResults,
@@ -86,7 +86,7 @@ func writeOrDone(ctx context.Context, ch chan<- interface{}, item interface{}) b
 // connection over the provided channel. This effectively allows a select
 // statement to run on conn.Receive() (by selecting on outChan that is
 // passed into this function)
-func receiveMessages(ctx context.Context, conn *redigo.PubSubConn, outChan chan<- interface{}, logger log.Logger) {
+func receiveMessages(ctx context.Context, conn *redigo.PubSubConn, outChan chan<- any, logger *logging.Logger) {
 	defer close(outChan)
 
 	for {
@@ -136,7 +136,7 @@ func (r *redisQueryResults) ReadChannel(ctx context.Context, query fleet.Distrib
 
 	var wg sync.WaitGroup
 
-	logger := log.With(r.logger, "campaignID", query.ID)
+	logger := r.logger.With("campaignID", query.ID)
 
 	// Run a separate goroutine feeding redis messages into msgChannel.
 	wg.Add(+1)

server/pubsub/testing_utils.go

@@ -4,11 +4,11 @@ import (
 	"testing"
 
 	"github.com/fleetdm/fleet/v4/server/datastore/redis/redistest"
-	"github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 )
 
 func SetupRedisForTest(t *testing.T, cluster, readReplica bool) *redisQueryResults {
 	const dupResults = false
 	pool := redistest.SetupRedis(t, "zz", cluster, false, readReplica)
-	return NewRedisQueryResults(pool, dupResults, log.NewNopLogger())
+	return NewRedisQueryResults(pool, dupResults, logging.NewNopLogger())
 }