[BUG] AgentScopeRoutingMergeNode should not re-synthesize single routed result

### Is there an existing issue for this? - [x] I have searched the existing issues ### Current Behavior `AgentScopeRoutingMergeNode` always calls `synthesize()` after collecting routed sub-agent outputs, even when only one sub-agent produced a result. This is inconsistent with the regular `RoutingMergeNode`, which already returns a single routed result directly without another synthesis model call. As a result, a single sub-agent answer may be sent back to the model for an unnecessary synthesis step, which can increase latency/token cost and may rewrite or duplicate the original answer. ### Expected Behavior When only one sub-agent result is present, `AgentScopeRoutingMergeNode` should return that result directly, without invoking `synthesize()`. The synthesis step should only be used when multiple sub-agent results need to be merged. ### Steps To Reproduce 1. Build an AgentScope routing agent with multiple sub-agents. 2. Send a request that is routed to only one sub-agent. 3. The selected sub-agent writes its result to state. 4. `AgentScopeRoutingMergeNode` collects exactly one result. 5. Observe that it still calls `synthesize()` and creates a `ReActAgent` for another model call instead of returning the single result directly. ### Environment ```markdown Spring AI Alibaba version(s): latest main branch ``` ### Debug logs _No response_ ### Anything else? The regular `RoutingMergeNode` already has a guard for this case: ```java if (formattedResults.size() == 1) { return Map.of(mergedOutputKey, lastResult); } ``` `AgentScopeRoutingMergeNode` does not have the same guard and always executes: ```java String query = extractOriginalQuery(state); String finalAnswer = synthesize(query, formattedResults); return Map.of(mergedOutputKey, finalAnswer); ``` Suggested fix: * Track the collected single result. * Return it directly when formattedResults.size() == 1. * Keep the existing synthesis behavior for multiple results. * Add a regression test for the single-result case.