fix(sessions): keep the lineage project id when the root is the freshest segment

compression_tip() resolves a project assignment across the whole lineage, so it
can find one recorded on a newer EMPTY continuation. When the root is still the
freshest *importable* segment, the `tip is row` early return appended the root
without applying that resolved id and the assignment was silently dropped.

Copy the root and apply lineage_project_id before appending, matching what the
merged-tip path below already does.

api/agent_sessions.py

@@ -614,7 +614,14 @@ def _project_agent_session_rows(rows: list[dict]) -> list[dict]:
             continue
 
         if tip is row:
-            projected.append(dict(row))
+            # The root can still be the freshest *importable* segment while a
+            # newer EMPTY continuation carries the project assignment. Applying
+            # the resolved lineage id here too keeps that assignment instead of
+            # silently dropping it on the early-return path (#6659).
+            root_only = dict(row)
+            if lineage_project_id:
+                root_only['project_id'] = lineage_project_id
+            projected.append(root_only)
             continue
 
         merged = dict(row)