fix: close documented platform polish gaps

api/src/channels/channels.service.spec.ts

@@ -279,7 +279,7 @@ describe('ChannelsService', () => {
     };
 
     await expect(service.createChannel({ ...channel, rateLimitPerSecond: 2001 })).rejects.toThrow('rateLimitPerSecond must be between 1 and 2000');
-    await expect(service.createChannel({ ...channel, config: { extensionDigits: 3 } })).rejects.toThrow('extensionDigits must be one of 0, 2, 4, or 6');
+    await expect(service.createChannel({ ...channel, config: { extensionDigits: 21 } })).rejects.toThrow('extensionDigits must be an integer between 0 and 20');
   });
 
   it('updates CMPP channel configuration without requiring password changes', async () => {
@@ -326,6 +326,17 @@ describe('ChannelsService', () => {
     });
   });
 
+  it('persists an arbitrary integer extension digit count within the supported range', async () => {
+    const prisma = createPrismaMock();
+    const service = new ChannelsService(prisma as never);
+
+    await service.updateChannel('channel-1', { config: { extensionDigits: 15 } });
+
+    expect(prisma.smsChannel.update).toHaveBeenCalledWith(expect.objectContaining({
+      data: expect.objectContaining({ config: expect.objectContaining({ extensionDigits: 15 }) }),
+    }));
+  });
+
   it('rejects invalid channel update ports', async () => {
     const prisma = createPrismaMock();
     const service = new ChannelsService(prisma as never);