fix: harden multi-model chat selection

frontend/src/lib/consultation-model-selection.ts

@@ -0,0 +1,18 @@
+export async function reserveConsultationModel<
+  Model extends { readonly id: string },
+  Reservation,
+>(
+  modelId: string,
+  resolveModel: (modelId: string) => Model | null,
+  reserveCredit: () => Promise<Reservation>,
+) {
+  const model = resolveModel(modelId);
+  if (!model) return { status: "unavailable" } as const;
+
+  return {
+    status: "reserved",
+    model,
+    usageModelId: model.id,
+    reservation: await reserveCredit(),
+  } as const;
+}

frontend/src/lib/session-model-persistence.ts

@@ -0,0 +1,34 @@
+type SessionModelWrite = {
+  readonly values: { readonly model_id: string };
+  readonly sessionId: string;
+  readonly userId: string;
+};
+
+type SessionModelWriter = (write: SessionModelWrite) => PromiseLike<{
+  found: boolean;
+  error: string | null;
+}>;
+
+export async function persistSessionModelSelection(
+  write: SessionModelWriter,
+  userId: string,
+  sessionId: string,
+  modelId: string,
+) {
+  const result = await write({ values: { model_id: modelId }, sessionId, userId });
+  if (result.error) throw new Error(`云端同步失败：${result.error}`);
+  if (!result.found) throw new Error("云端同步失败：对话不存在或无权修改");
+}
+
+export class SessionModelPersistenceQueue {
+  private readonly pending = new Map<string, Promise<void>>();
+
+  enqueue(sessionId: string, write: () => Promise<void>) {
+    const previous = this.pending.get(sessionId) ?? Promise.resolve();
+    const current = previous.catch(() => undefined).then(write);
+    this.pending.set(sessionId, current);
+    return current.finally(() => {
+      if (this.pending.get(sessionId) === current) this.pending.delete(sessionId);
+    });
+  }
+}