[mdns] Drive event-driven polling from Ethernet IP state events on RP2040

Extends the WiFi-only listener pattern from the previous commit to also subscribe
to EthernetIPStateListener when Ethernet is configured. RP2040 can run mDNS over a
W5500 ethernet shield without WiFi, and mDNS and WiFi are mutually exclusive on
RP2040 (the framework doesn't support both simultaneously on the CYW43/PIO paths),
so this adds the ethernet-only path without touching the WiFi path.

ESPHome's wifi and ethernet components already publish compatible IP state listener
APIs (`WiFiIPStateListener::on_ip_state` and `EthernetIPStateListener::on_ip_state`
with identical signatures). MDNSComponent multiply-inherits both when available; a
single on_ip_state() override satisfies both vtable entries.

- New `USE_MDNS_WIFI_LISTENER` / `USE_MDNS_ETHERNET_LISTENER` gates control per-
  interface subscription. `USE_MDNS_EVENT_DRIVEN_POLLING` fires if either is
  available.
- Python side now calls `ethernet.request_ethernet_ip_state_listener()` when
  ethernet is in the config (RP2040 only — ESP8266 has no ethernet driver).
- setup() seeds current state for each registered listener so an already-up
  interface still triggers MDNS.begin() + polling window under AFTER_CONNECTION
  priority.

Tests: adds `test-enabled-ethernet.rp2040-ard.yaml` covering the ethernet-only
path. Existing `test-enabled.rp2040-ard.yaml` (WiFi-only) and ESP8266 tests
continue to pass.

tests/components/mdns/common-enabled-ethernet.yaml

@@ -0,0 +1,23 @@
+ethernet:
+  type: W5500
+  clk_pin: 18
+  mosi_pin: 19
+  miso_pin: 16
+  cs_pin: 17
+  interrupt_pin: 21
+  reset_pin: 20
+  manual_ip:
+    static_ip: 192.168.178.56
+    gateway: 192.168.178.1
+    subnet: 255.255.255.0
+  domain: .local
+  mac_address: "02:AA:BB:CC:DD:01"
+
+mdns:
+  disabled: false
+  services:
+    - service: _test_service
+      protocol: _tcp
+      port: 8888
+      txt:
+        static_string: Anything

tests/components/mdns/test-enabled-ethernet.rp2040-ard.yaml

@@ -0,0 +1 @@
+<<: !include common-enabled-ethernet.yaml