<!-- 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
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## 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 -->
