fix(assets): remove unused delete_content param from deleteAsset (#14241)

* fix(assets): remove unused delete_content param from deleteAsset

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.

* test/docs: address review on deleteAsset delete_content removal

- Rename test_delete_upon_reference_count ->
  test_soft_delete_preserves_asset_identity_across_references; the old name
  implied last-ref cleanup, but it now verifies the opposite (soft delete
  preserves identity across references).
- Strengthen the re-association assertion: also check asset_hash == src_hash
  so it proves content reuse rather than relying on the now-tautological
  created_new is False.
- Document delete_asset_reference: the orphan-reclamation branch is
  intentionally internal-only; the public endpoint always soft-deletes.
- Normalize the soft-delete comment phrasing.

* test(assets): make seed content unique per test for isolation

Removing the delete_content param means delete is always a soft delete, so
content created by one test now survives into the next. The suite had been
relying on hard-delete teardown for isolation, so shared fixed-content
fixtures started colliding: seeded_asset (b"A"*4096) and
make_asset_bytes (deterministic on name) produced the same hash every test,
so the second seed deduped to the surviving asset and returned 200 instead
of 201, cascading into ~14 failures/errors.

Salt both fixtures with a per-test uuid so each test creates fresh content
(created_new True, 201), while keeping content deterministic within a test
(same name/size -> same bytes) and preserving exact byte length so size-based
list/sort assertions are unaffected.

app/assets/api/routes.py

@@ -533,18 +533,14 @@ async def update_asset_route(request: web.Request) -> web.Response:
 @_require_assets_feature_enabled
 async def delete_asset_route(request: web.Request) -> web.Response:
     reference_id = str(uuid.UUID(request.match_info["id"]))
-    delete_content_param = request.query.get("delete_content")
-    delete_content = (
-        False
-        if delete_content_param is None
-        else delete_content_param.lower() not in {"0", "false", "no"}
-    )
 
     try:
+        # Deleting an asset is a soft delete of the reference; the underlying
+        # content is preserved (it may be shared with other references).
         deleted = delete_asset_reference(
             reference_id=reference_id,
             owner_id=USER_MANAGER.get_request_user_id(request),
-            delete_content_if_orphan=delete_content,
+            delete_content_if_orphan=False,
         )
     except Exception:
         logging.exception(

app/assets/services/asset_management.py

@@ -160,6 +160,16 @@ def delete_asset_reference(
     owner_id: str,
     delete_content_if_orphan: bool = True,
 ) -> bool:
+    """Delete an asset reference.
+
+    With ``delete_content_if_orphan=False`` (a soft delete), the reference is
+    hidden and the underlying content is preserved. With ``True``, the content
+    is also removed once it becomes orphaned.
+
+    Note: the public DELETE /api/assets/{id} endpoint always soft-deletes
+    (passes ``False``); the orphan-reclamation path is intentionally
+    internal-only, retained for a future GC/admin caller.
+    """
     with create_session() as session:
         if not delete_content_if_orphan:
             # Soft delete: mark the reference as deleted but keep everything