[routing-manager] delay sending RAs until after initial policy evaluation (#9896)

This commit modifies the `RoutingManager` to not send any Router
Advertisements (RAs) until the initial policy evaluation has
completed.

This change leverages the `IsInitialPolicyEvaluationDone()` method
which ensures that both OMR and on-link prefixes have been
determined. Specifically, this guarantees that we wait for the
initial set of Router Solicitation (RS) messages to be sent and that
we discover all other routers on the AIL before sending the first RA.
This is because on-link prefix evaluation itself is dependent on the
`mRsSender.IsInProgress()` check that waits till all RS messages are
sent and RAs from other routers are received and processed.

Without this change, the BR could receive its own RS message,
triggering it to send an RA prematurely, potentially with incorrect
M and O flags.

src/core/border_router/routing_manager.cpp

@@ -478,7 +478,10 @@ void RoutingManager::EvaluateRoutingPolicy(void)
     mNat64PrefixManager.Evaluate();
 #endif
 
-    SendRouterAdvertisement(kAdvPrefixesFromNetData);
+    if (IsInitalPolicyEvaluationDone())
+    {
+        SendRouterAdvertisement(kAdvPrefixesFromNetData);
+    }
 
 #if OPENTHREAD_CONFIG_SRP_SERVER_ENABLE
     if (Get<Srp::Server>().IsAutoEnableMode() && IsInitalPolicyEvaluationDone())