## Why
Clients need to correlate a remote plugin installation request with a specific
installation attempt.
## What changed
- Add the optional `installAttemptId` field to `PluginInstallParams`.
- Forward the value as `install_attempt_id` in the remote plugin install POST
body.
- Preserve the existing empty request body when the field is omitted.
## Testing
- Cover protocol serialization and remote install request bodies with and
without an attempt ID.
GitOrigin-RevId: ffdceff4f9329b650848e6428a9ba39646c242c3