server: cap shared decision prefixes at the first output (#30710)

Limit the shared prefix to the first required output position of the parent and each child task.
This keeps decision output embeddings available in the final batch when questions share option tokens.

Fixes #30482

Signed-off-by: aiqubits <aiqubits@hotmail.com>

tools/server/server-decision.cpp

@@ -1762,7 +1762,7 @@ std::vector<server_task> server_decision_group_tasks(std::vector<server_task> &&
         size_t n_shared = parent.tokens.size() - 1;
         for (size_t j = i + 1; j < end; j++) {
             n_shared = std::min(n_shared, parent.tokens.get_common_prefix(tasks[j].tokens));
-            n_shared = std::min(n_shared, tasks[j].tokens.size() - 1);
+            n_shared = std::min({n_shared, tasks[j].tokens.size() - 1, (size_t) parent.decision.pos_first(), (size_t) tasks[j].decision.pos_first()});
         }
 
         if (end - i < 2 || n_shared == 0) {