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/service/testing_utils.go

@@ -23,7 +23,6 @@ import (
 	"github.com/fleetdm/fleet/v4/ee/server/service/est"
 	"github.com/fleetdm/fleet/v4/ee/server/service/hostidentity"
 	"github.com/fleetdm/fleet/v4/ee/server/service/hostidentity/httpsig"
-	"github.com/fleetdm/fleet/v4/server"
 	"github.com/fleetdm/fleet/v4/server/acl/activityacl"
 	activity_api "github.com/fleetdm/fleet/v4/server/activity/api"
 	activity_bootstrap "github.com/fleetdm/fleet/v4/server/activity/bootstrap"
@@ -491,9 +490,6 @@ func RunServerForTestsWithServiceWithDS(t *testing.T, ctx context.Context, ds fl
 			opts[0].DBConns,
 			activityAuthorizer,
 			activityACLAdapter,
-			func(ctx context.Context, url string, payload any) error {
-				return server.PostJSONWithTimeout(ctx, url, payload, slogLogger)
-			},
 			slogLogger,
 		)
 		svc.SetActivityService(activitySvc)