Move PostJSONWithTimeout to platform/http package and activity cleanup (#40561)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #38536

- Moved PostJSONWithTimeout to platform/http
- Created platform/errors package with only types needed by ctxerr. This
way, ctxerr did not need to import fleethttp.
- Made activity bounded context use PostJSONWithTimeout directly
- Removed some activity types from legacy code that were no longer
needed

# Checklist for submitter

- [ ] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
- Changes file `38536-new-activity-bc` already present, and this is just
cleanup from that work.

## 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

## Release Notes

* **Refactor**
* Reorganized error handling utilities for improved clarity and
decoupling.
* Consolidated HTTP utilities to centralize JSON posting functionality
with timeout support.
* Simplified activity service initialization by removing unused internal
parameters.
* Cleaned up test utilities and removed webhook-related test
scaffolding.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

server/contexts/ctxerr/ctxerr.go

@@ -21,7 +21,7 @@ import (
 	"strings"
 	"time"
 
-	platform_http "github.com/fleetdm/fleet/v4/server/platform/http"
+	platform_errors "github.com/fleetdm/fleet/v4/server/platform/errors"
 	"github.com/getsentry/sentry-go"
 	"go.elastic.co/apm/v2"
 	"go.opentelemetry.io/otel/attribute"
@@ -202,7 +202,7 @@ func Wrapf(ctx context.Context, cause error, format string, args ...interface{})
 
 // Cause returns the root error in err's chain.
 func Cause(err error) error {
-	return platform_http.Cause(err)
+	return platform_errors.Cause(err)
 }
 
 // FleetCause is similar to Cause, but returns the root-most
@@ -407,7 +407,7 @@ func isClientError(err error) bool {
 	// Check for explicit client error interface. All 4xx error types
 	// (not found, already exists, conflict, validation, permission,
 	// bad request, foreign key, etc.) should implement this interface.
-	var clientErr platform_http.ErrWithIsClientError
+	var clientErr platform_errors.ErrWithIsClientError
 	if errors.As(err, &clientErr) {
 		return clientErr.IsClientError()
 	}

server/contexts/logging/logging.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 	"time"
 
+	platform_errors "github.com/fleetdm/fleet/v4/server/platform/errors"
 	platform_http "github.com/fleetdm/fleet/v4/server/platform/http"
 	kithttp "github.com/go-kit/kit/transport/http"
 )
@@ -233,7 +234,7 @@ func (l *LoggingContext) setLevelError() bool {
 	}
 
 	if len(l.Errs) == 1 {
-		var ew platform_http.ErrWithIsClientError
+		var ew platform_errors.ErrWithIsClientError
 		if errors.As(l.Errs[0], &ew) && ew.IsClientError() {
 			return false
 		}