test: exercise compact parent projection

tests/test_465_session_branching.py

@@ -599,20 +599,12 @@ def test_session_model_parent_session_id():
 
 
 def test_session_compact_includes_parent():
-    """Verify compact() includes parent_session_id."""
-    src = _read('api/models.py')
-    # Find the compact method and scan its full body for parent_session_id.
-    # PR #1591 (May 2026) added a has_pending_user_message recompute block at
-    # the top of compact() which pushed the parent_session_id field beyond a
-    # 1500-char window — widen the scan to 3000 chars to cover the full
-    # return-dict body without re-tightening every time compact() grows.
-    # compact() accepts optional projection flags on separate lines; match the
-    # method name and first parameter without pinning formatting.
-    compact_def_match = re.search(r"def compact\(\s*self", src)
-    assert compact_def_match, "Could not find compact() method"
-    snippet = src[compact_def_match.start():compact_def_match.start() + 3500]
-    assert "'parent_session_id'" in snippet, \
-        "compact() should include parent_session_id"
+    """Verify compact() exposes a fork's parent and omits it for root sessions."""
+    fork = routes.Session(session_id="fork-1", parent_session_id="parent-1")
+    assert fork.compact()["parent_session_id"] == "parent-1"
+
+    root = routes.Session(session_id="root-1")
+    assert "parent_session_id" not in root.compact()
 
 
 def test_session_metadata_fields_includes_parent():