[wifi] Deprecate wifi_ssid() in favor of wifi_ssid_to()

wifi_ssid() returns std::string which allocates on the heap.
All internal callers have already been migrated to the
buffer-based wifi_ssid_to() alternative. Mark as deprecated
with a 6-month removal window (2026.9.0).

esphome/components/wifi/wifi_component.h

@@ -502,6 +502,8 @@ class WiFiComponent : public Component {
   }
 
   network::IPAddresses wifi_sta_ip_addresses();
+  // Remove before 2026.9.0
+  ESPDEPRECATED("Use wifi_ssid_to() instead. Removed in 2026.9.0", "2026.3.0")
   std::string wifi_ssid();
   /// Write SSID to buffer without heap allocation.
   /// Returns pointer to buffer, or empty string if not connected.