fix(sip): send 487 Request Terminated after OK (CANCEL) for pending INVITE (#372)

When a CANCEL is received for an INVITE that hasn't been answered,
the SIP gateway now correctly sends a 487 response to the original
INVITE. This ensures proper SIP transaction termination and avoids
issues where upstream proxies (e.g., Brekeke) keep the call in
an INVITE state due to the missing final response.

Author: mubashiroliyantakath

pkg/sip/inbound.go

@@ -1161,9 +1161,7 @@ func (c *sipInbound) StartRinging() {
 			case r := <-cancels:
 				close(c.cancelled)
 				_ = tx.Respond(sip.NewResponseFromRequest(r, sip.StatusOK, "OK", nil))
-				c.mu.Lock()
-				c.drop()
-				c.mu.Unlock()
+				c.RespondAndDrop(sip.StatusRequestTerminated, "Request Terminated")
 				return
 			case <-ticker.C:
 			}