[mdns] Simplify listener logic: always re-arm on IP notify, drop transition tracking

ESPHome's WiFiIPStateListener only notifies on IP acquisition (GOT_IP events), not
on IP loss — on disconnect, only the WiFiConnectStateListener's disconnect path
fires (see wifi_component_esp8266.cpp:952-962 and wifi_component_pico_w.cpp:340).

The previous commit's `ip_was_up_` transition tracking was broken: after the first
IP-up event, `ip_was_up_` latched to true and never reset, so subsequent
disconnect+reconnect cycles would see has_ip=true && ip_was_up_=true and skip
re-arming the polling window.

Fix: always re-arm on any IP notification. The scheduler's set_interval/set_timeout
with a uint32_t ID already performs atomic cancel-and-add for matching IDs
(Scheduler::set_timer_common_ line 232-234), so start_polling_window_ is idempotent
and needs no explicit cancel. Drop the ip_was_up_ field and cancel_polling_window_
helper entirely.

The !has_ip branch (cancel on disconnect) was dead code: it would never fire because
the listener doesn't receive disconnect events. Removing it; the polling window will
naturally expire on its own (at most 12s of harmless MDNS.update() calls during a
disconnect that isn't followed by reconnect within the window).

esphome/components/mdns/mdns_component.cpp

@@ -192,16 +192,12 @@ void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUN
 
 #ifdef USE_MDNS_EVENT_DRIVEN_POLLING
 void MDNSComponent::start_polling_window_() {
-  // Re-arming replaces the previous window; cancel any active schedulers first.
-  this->cancel_polling_window_();
+  // Re-arming replaces the previous window. The scheduler's set_interval/set_timeout
+  // with a uint32_t ID already does atomic cancel-and-add for items sharing that ID
+  // (see Scheduler::set_timer_common_), so no explicit cancel is needed.
   this->set_interval(MDNS_POLL_ID, MDNS_UPDATE_INTERVAL_MS, mdns_pump_update);
   this->set_timeout(MDNS_POLL_STOP_ID, MDNS_POLL_WINDOW_MS, [this]() { this->cancel_interval(MDNS_POLL_ID); });
 }
-
-void MDNSComponent::cancel_polling_window_() {
-  this->cancel_interval(MDNS_POLL_ID);
-  this->cancel_timeout(MDNS_POLL_STOP_ID);
-}
 #endif
 
 void MDNSComponent::dump_config() {

esphome/components/mdns/mdns_component.h

@@ -122,9 +122,6 @@ class MDNSComponent final : public Component
   /// Arm a bounded polling window so MDNS.update() runs at MDNS_UPDATE_INTERVAL_MS
   /// for MDNS_POLL_WINDOW_MS. A subsequent call replaces the previous window.
   void start_polling_window_();
-  /// Cancel any active polling window.
-  void cancel_polling_window_();
-  bool ip_was_up_{false};
 #endif
   /// Helper to set up services and MAC buffers, then call platform-specific registration
   using PlatformRegisterFn = void (*)(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &);

esphome/components/mdns/mdns_esp8266.cpp

@@ -51,16 +51,14 @@ void MDNSComponent::setup() {
 
 void MDNSComponent::on_ip_state(const network::IPAddresses &ips, const network::IPAddress &,
                                 const network::IPAddress &) {
-  const bool has_ip = ips[0].is_set();
-  if (has_ip && !this->ip_was_up_) {
-    // IP came up. LEAmDNS's internal lwIP callback will call _restart() shortly after
-    // (if it hasn't already) — arm the polling window so the probe/announce phase is
-    // serviced regardless of our relative timing vs the library's callback.
+  // ESPHome's WiFiIPStateListener only notifies on IP acquisition (GOT_IP events on
+  // ESP8266 — see wifi_component_esp8266.cpp), not on IP loss, so every notification
+  // represents a fresh IP that the LEAmDNS library's lwIP callback will trigger a
+  // _restart() for. Always re-arm the polling window — start_polling_window_() is
+  // idempotent (scheduler does atomic cancel-and-add on matching IDs).
+  if (ips[0].is_set()) {
     this->start_polling_window_();
-  } else if (!has_ip && this->ip_was_up_) {
-    this->cancel_polling_window_();
   }
-  this->ip_was_up_ = has_ip;
 }
 
 void MDNSComponent::on_shutdown() {

esphome/components/mdns/mdns_rp2040.cpp

@@ -91,19 +91,21 @@ void MDNSComponent::setup() {
 #ifdef USE_MDNS_EVENT_DRIVEN_POLLING
 void MDNSComponent::on_ip_state(const network::IPAddresses &ips, const network::IPAddress &,
                                 const network::IPAddress &) {
-  const bool has_ip = ips[0].is_set();
-  if (has_ip && !this->ip_was_up_) {
-    if (!this->initialized_) {
-      this->setup_buffers_and_register_(register_rp2040);
-      this->initialized_ = true;
-    } else {
-      MDNS.notifyAPChange();
-    }
-    this->start_polling_window_();
-  } else if (!has_ip && this->ip_was_up_) {
-    this->cancel_polling_window_();
+  // ESPHome's WiFiIPStateListener only notifies on IP acquisition (see
+  // wifi_component_pico_w.cpp), not on IP loss, so every notification represents a
+  // fresh IP that needs a probe/announce cycle. The library's internal
+  // LwipIntf::stateUpCB is stubbed out on arduino-pico (see setup()), so we drive
+  // begin/restart ourselves from this callback.
+  if (!ips[0].is_set()) {
+    return;
   }
-  this->ip_was_up_ = has_ip;
+  if (!this->initialized_) {
+    this->setup_buffers_and_register_(register_rp2040);
+    this->initialized_ = true;
+  } else {
+    MDNS.notifyAPChange();
+  }
+  this->start_polling_window_();
 }
 #endif