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/logging/nanodep.go

@@ -2,16 +2,16 @@ package logging
 
 import (
 	nanodep_log "github.com/fleetdm/fleet/v4/server/mdm/nanodep/log"
-	kitlog "github.com/go-kit/log"
+	"github.com/fleetdm/fleet/v4/server/platform/logging"
 	"github.com/go-kit/log/level"
 )
 
 // NanoDEPLogger is a logger adapter for nanodep.
 type NanoDEPLogger struct {
-	logger kitlog.Logger
+	logger *logging.Logger
 }
 
-func NewNanoDEPLogger(logger kitlog.Logger) *NanoDEPLogger {
+func NewNanoDEPLogger(logger *logging.Logger) *NanoDEPLogger {
 	return &NanoDEPLogger{
 		logger: logger,
 	}
@@ -26,8 +26,7 @@ func (l *NanoDEPLogger) Debug(keyvals ...interface{}) {
 }
 
 func (l *NanoDEPLogger) With(keyvals ...interface{}) nanodep_log.Logger {
-	newLogger := kitlog.With(l.logger, keyvals...)
 	return &NanoDEPLogger{
-		logger: newLogger,
+		logger: l.logger.With(keyvals...),
 	}
 }