[wifi] Defer esp_wifi_init() to lazy-init so enable_on_boot: false actually saves RAM (#16606)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/components/wifi/test-lifecycle.esp32-idf.yaml

@@ -0,0 +1,15 @@
+wifi:
+  ssid: MySSID
+  password: password1
+  enable_on_boot: false
+
+esphome:
+  on_boot:
+    priority: 200
+    then:
+      - if:
+          condition:
+            not:
+              wifi.enabled:
+          then:
+            - wifi.enable: