[espnow] Validate radio-less variants need esp32_hosted; add P4 compile test

ESP-NOW rides the Wi-Fi PHY, so on a radio-less esp32 variant the espnow
component would otherwise fail with an inscrutable "undefined reference to
esp_now_*" at link time. Fail fast in final validation instead: the P4 must
have the esp32_hosted shim, and other radio-less variants have no ESP-NOW
path at all. Adds a P4 esp32_hosted + espnow compile test and unit tests for
the new validation.

tests/components/esp32_hosted/test-espnow.esp32-p4-idf.yaml

@@ -0,0 +1,5 @@
+# Exercises the ESP-NOW-over-hosted shim: on the ESP32-P4 host, esp32_hosted
+# supplies the esp_now_* symbols that the espnow component links against.
+packages:
+  esp32_hosted: !include common.yaml
+  espnow: !include ../espnow/common.yaml

tests/unit_tests/components/test_espnow.py

@@ -0,0 +1,48 @@
+"""Tests for the espnow component's final validation."""
+
+import pytest
+
+from esphome.components.esp32.const import (
+    VARIANT_ESP32C3,
+    VARIANT_ESP32H2,
+    VARIANT_ESP32P4,
+)
+from esphome.components.espnow import _validate_variant
+import esphome.config_validation as cv
+import esphome.final_validate as fv
+from esphome.types import ConfigType
+
+
+def _run(
+    monkeypatch, variant: str, full_config: dict, config: ConfigType
+) -> ConfigType:
+    monkeypatch.setattr("esphome.components.espnow.get_esp32_variant", lambda: variant)
+    token = fv.full_config.set(full_config)
+    try:
+        return _validate_variant(config)
+    finally:
+        fv.full_config.reset(token)
+
+
+def test_variant_with_native_wifi_passes(monkeypatch) -> None:
+    """A variant with a native Wi-Fi PHY needs no shim; config passes through."""
+    config = {"id": "espnow"}
+    assert _run(monkeypatch, VARIANT_ESP32C3, {}, config) is config
+
+
+def test_radioless_non_p4_variant_rejected(monkeypatch) -> None:
+    """Radio-less variants without any ESP-NOW path are rejected outright."""
+    with pytest.raises(cv.Invalid, match="not supported"):
+        _run(monkeypatch, VARIANT_ESP32H2, {}, {})
+
+
+def test_p4_without_esp32_hosted_rejected(monkeypatch) -> None:
+    """The P4 needs the esp32_hosted shim to supply the esp_now_* symbols."""
+    with pytest.raises(cv.Invalid, match="esp32_hosted"):
+        _run(monkeypatch, VARIANT_ESP32P4, {}, {})
+
+
+def test_p4_with_esp32_hosted_passes(monkeypatch) -> None:
+    """The P4 with esp32_hosted present validates; config passes through."""
+    config = {"id": "espnow"}
+    assert _run(monkeypatch, VARIANT_ESP32P4, {"esp32_hosted": {}}, config) is config