fix: escape Telegram MarkdownV2 after template rendering

[GivenBY]

📝 Summary

This pull request fixes telegram notification failures when using MarkdownV2 together with custom message templates.

currently, template variables such as {{ hostnameOrURL }}, {{ monitorJSON }}, or {{ heartbeatJSON }} may contain characters reserved by telegram MarkdownV2 syntax (e.g. ., -, _, {}, etc.). These characters are not escaped, causing telegram API errors and making templates effectively unusable.

This change ensures that when MarkdownV2 is selected, all template values are safely escaped before sending the message to telegram. This removes the need for users to manually escape content they do not control and aligns behavior with telegram MarkdownV2 specification.

The change is scoped only to the telegram provider and does not affect other providers or parse modes.

📋 Related issues

Resolves #6549

📄 Checklist

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 have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🛠️ I have reviewed and tested my code.
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
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• 🧰 Dependency updates are listed and explained.

[CommanderStorm]

a second renderTemplate in the telegram monitor is a non-starter for us.
Please remove this and pull only the escaping into the telgram notification provider instead.

Also, plese share a screenshot of this working (with 100s of notification providers, I cannot do this myself and you have already tested this)

[iotux]

a second renderTemplate in the telegram monitor is a non-starter for us. Please remove this and pull only the escaping into the telgram notification provider instead.

Also, plese share a screenshot of this working (with 100s of notification providers, I cannot do this myself and you have already tested this)

It looks like double work going on with this:
https://github.com/iotux/uptime-kuma/blob/fix/markdownv2/server/notification-providers/telegram.js

The test template:

*Alarm*
Name: {{ name }}
Status: {{ status }}
Hostname: {{ hostnameOrURL }}
Mon name: {{ monitorJSON.name }}
Mon hostname: {{ monitorJSON.hostName }}
Time: {{ heartbeatJSON.time }}
Message: {{ msg }}

The screenshots:

[GivenBY]

@iotux
The issue still reproduces during testing because the default values are injected upstream by NotificationProvider, before the Telegram provider is invoked.

With the latest changes applied, MarkdownV2 escaping now works correctly

Test Template:

*Alarm*
Name: {{ name }}
Status: {{ status }}
Hostname: {{ hostnameOrURL }}
Mon name: {{ monitorJSON.name }}
Mon hostname: {{ monitorJSON.hostName }}
Time: {{ heartbeatJSON.time }}
Message: {{ msg }}

The Screenshots:

[CommanderStorm]

LGTM

[CommanderStorm]

Thanks for working on fixing this issue you two ❤️