[mdns] Enforce network-interface availability at config time on ESP8266/RP2040

Addresses Copilot review feedback: if someone enables mdns on ESP8266 or RP2040
without wifi (or ethernet on RP2040), the listener-based setup() is a no-op and
the user sees a silent failure rather than a helpful error.

FINAL_VALIDATE_SCHEMA rejects mdns on these platforms when no compatible
network component is present, naming the specific options that would satisfy
the requirement. The existing DEPENDENCIES = ["network"] covers most
misconfigurations indirectly, but an explicit network: alone (without wifi or
ethernet) slips past that check — now it fails with a clear message.

esphome/components/mdns/__init__.py

@@ -14,6 +14,7 @@ from esphome.const import (
 from esphome.core import CORE, Lambda, coroutine_with_priority
 from esphome.coroutine import CoroPriority
 from esphome.cpp_generator import LambdaExpression
+import esphome.final_validate as fv
 from esphome.types import ConfigType
 
 CODEOWNERS = ["@esphome/core"]
@@ -61,6 +62,29 @@ def _consume_mdns_sockets(config: ConfigType) -> ConfigType:
     return config
 
 
+def _require_network_interface(config: ConfigType) -> ConfigType:
+    """Require a network interface for mDNS on Arduino/LEAmDNS platforms.
+
+    On ESP8266 and RP2040 the C++ implementation needs at least one IP state
+    listener (WiFi on ESP8266; WiFi or Ethernet on RP2040) to arm its polling
+    window. Reject at config time rather than silently producing a component
+    that never initializes.
+    """
+    if config.get(CONF_DISABLED):
+        return config
+    if not CORE.using_arduino or not (CORE.is_esp8266 or CORE.is_rp2040):
+        return config
+    full_config = fv.full_config.get()
+    has_wifi = "wifi" in full_config
+    has_ethernet = CORE.is_rp2040 and "ethernet" in full_config
+    if not (has_wifi or has_ethernet):
+        options = "'wifi'" if CORE.is_esp8266 else "'wifi' or 'ethernet'"
+        raise cv.Invalid(
+            f"mdns on this platform requires a network interface — add a {options} component to your configuration."
+        )
+    return config
+
+
 CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
@@ -74,6 +98,9 @@ CONFIG_SCHEMA = cv.All(
 )
 
 
+FINAL_VALIDATE_SCHEMA = _require_network_interface
+
+
 def mdns_txt_record(key: str, value: str) -> cg.RawExpression:
     """Create a mDNS TXT record.