fix(chat): make task session rendering authoritative

Use the Task part's state.metadata.sessionId as the only live identity contract for child sessions. Remove timestamp, status, and ordering heuristics that could misassociate concurrent subagents or repeatedly scan directory sessions while parts stream.

Extract Task metadata parsing and child-summary projection into a focused model with cached projections for unchanged message records. Preserve output and part-level metadata parsing only for legacy persisted records, and keep standalone Task rows visible when sorted activity groups are rendered.

Validated with focused Task and turn projection tests, UI type-check, lint, and dead-code analysis.

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -1839,7 +1839,7 @@ const AssistantMessageBody = React.memo(({
                 }
 
                 const activity = activityByPart.get(part);
-                if (activity?.kind === 'tool' && (shouldRenderActivityGroup || !isStandaloneTool(toolName))) {
+                if (activity?.kind === 'tool' && !isStandaloneTool(toolName)) {
                     i += 1;
                     continue;
                 }