fix(ingest): Save attachments without an event cache key (#126156)

Save and clean up event attachments even when the event payload is
passed inline without a `cache_key`. The save task now extracts
attachments based on `has_attachments`; their cache keys and Objectstore
references already travel in `_attachments`.

This prepares attachment handling for inline event payloads while
preserving cached attachments in self-hosted installations. Ingest
consumers and processing tasks already resolve attachment data
independently of the main event cache key.