webui : store reasoning_content so it is sent back in subsequent requests (#21249)

tools/server/webui/src/lib/stores/agentic.svelte.ts

@@ -474,6 +474,7 @@ class AgenticStore {
 			sessionMessages.push({
 				role: MessageRole.ASSISTANT,
 				content: turnContent || undefined,
+				reasoning_content: turnReasoningContent || undefined,
 				tool_calls: normalizedCalls
 			});