[WIP] Fix NPE in WebPush notification handling

[Claude]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>com.vaadin.flow.server.webpush.WebPush.java#L125 is NPE ("response" is null) instead of WebPushException</issue_title>
<issue_description>### Description of the bug

The line of code https://github.com/vaadin/flow/blob/main/flow-webpush/src/main/java/com/vaadin/flow/server/webpush/WebPush.java#L125
If SubscriptionState status == SubscriptionState.EXPIRED we should throw new WebPushException(
"Sending of web push notification failed with status code 404 or 410");
But instead of WebPushException we catch following Exception :
java.lang.NullPointerException: Cannot invoke "java.net.http.HttpResponse.body()" because "response" is null at com.vaadin.flow.server.webpush.WebPush.sendNotification(WebPush.java:125) ~[flow-webpush-25.1.3.jar:na] at digital.equinox.springcore.data.service.webpush.WebPushServiceImpl.notify(WebPushService.kt:83) ~[classes/:na]

Expected behavior

throw new WebPushException(
"Sending of web push notification failed with status code 404 or 410");

Minimal reproducible example

Just wait until WebPushSubscription is SubscriptionState.EXPIRED state

Versions

• Vaadin / Flow version: 25.1.3
• Java version: 21
• OS version: MacOS Tahoe 26.2 (25C56)
• Browser version (if applicable): Chrome Version 147.0.7727.138 (Official Build) (arm64)
• Application Server (if applicable): TomCat
• IDE (if applicable): IntelliJ IDEA 2025.3

</issue_description>

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