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.

tests-unit/assets_test/conftest.py

@@ -6,6 +6,7 @@ import subprocess
 import sys
 import tempfile
 import time
+import uuid
 from pathlib import Path
 from typing import Callable, Iterator, Optional
 
@@ -188,9 +189,17 @@ def _post_multipart_asset(
 
 @pytest.fixture
 def make_asset_bytes() -> Callable[[str, int], bytes]:
+    # Salt content per test so it never collides with assets left over from
+    # earlier tests. Delete is now always a soft delete (content is preserved),
+    # so the suite can no longer rely on hard-deleting content for isolation.
+    # Deterministic within a test: the same (name, size) yields the same bytes.
+    salt = uuid.uuid4().bytes
+
     def _make(name: str, size: int = 8192) -> bytes:
         seed = sum(ord(c) for c in name) % 251
-        return bytes((i * 31 + seed) % 256 for i in range(size))
+        body = bytearray((i * 31 + seed) % 256 for i in range(size))
+        body[: len(salt)] = salt[:size]
+        return bytes(body)
     return _make
 
 
@@ -227,7 +236,11 @@ def seeded_asset(request: pytest.FixtureRequest, http: requests.Session, api_bas
     if tags is None:
         tags = ["models", "checkpoints", "unit-tests", "alpha"]
     meta = {"purpose": "test", "epoch": 1, "flags": ["x", "y"], "nullable": None}
-    files = {"file": (name, b"A" * 4096, "application/octet-stream")}
+    # Unique content per test so the seed always creates a fresh asset (201).
+    # Delete is now always a soft delete, so content from a prior test survives
+    # and would otherwise dedup this upload into an existing asset (200).
+    content = uuid.uuid4().bytes + b"A" * (4096 - 16)
+    files = {"file": (name, content, "application/octet-stream")}
     form_data = {
         "tags": json.dumps(tags),
         "name": name,