fix(ntfy): check if the message was really accepted

[dharunashokkumar]

Summary

the test button showed success for any 2** response, so when a proxy redirected the publish to a login page it still said sent successfully but nothing arrived.

ntfy returns the created message when a publish works, so fail if there is no id in the response.

breaking change

setups whose ntfy server url redirects will now report an
error instead of a false success. That is the intent of the fix, but it will
surface as new errors for anyone currently affected.

testing

-- confirmed a real ntfy.sh publish returns "id": "...", ..., and that the
message actually arrive....
-- reproduced the bug with a local server that answers post with a 302 to a login
page: on master this reports "Sent Successfully.", with this change it fails.
-- added notification-providers/test-ntfy.js
--eslint and prettier done on both files

I used an AI assistant to navigate the codebase and draft this change. I have
reviewed and tested it myself.

Resolves #6050

yup, I used an ai to navigate the code and draft the change, and i have reviewed and test it my self perfectly, and i am responsible to explain evert line of code i submit

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
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  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

[github-actions]

Thanks for the PR! If anyone would like to help with testing, run: npx kuma-pr dharunashokkumar:fix/ntfy-test-verify-response (requires Node.js and Docker)