The delete_content query param on DELETE /api/assets/{id} was introduced
in #12125 and had its default flipped to false in #12621. In practice no
client sends it: the frontend issues a bare DELETE /assets/{id}, so every
real caller already gets the default soft-delete (the reference is hidden,
content preserved). The only thing that set delete_content=true was this
repo's own test teardown.
Remove the param from the route and the OpenAPI spec so the contract
matches what clients actually use (and lines up with the cloud surface).
The route now always soft-deletes. The underlying delete_asset_reference
helper keeps its delete_content_if_orphan option, so orphan reclamation
remains available internally for a future GC path — it's just no longer
exposed on the public endpoint. Tests that used delete_content=true for
hard cleanup now soft-delete; test_delete_upon_reference_count asserts
content preservation instead of orphan removal.
