fix: add callback validation in deleteMonitor handler

[kurama]

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📋 Overview

What problem does this pull request address?
This PR fixes an error in the deleteMonitor socket handler that causes the application to crash with TypeError: callback is not a function. This occurs when the callback parameter is undefined or not properly passed in certain deletion scenarios, particularly when deleting monitors.

Errir trace (on master branch):

2025-11-24T12:14:55+01:00 [DB] INFO: Delete Monitor completed in : 24 ms
Trace: TypeError: callback is not a function
    at Socket.<anonymous> (/home/dgrasset/dev/uptime-kuma/server/server.js:1083:17)
    at process.unexpectedErrorHandler (/home/dgrasset/dev/uptime-kuma/server/server.js:1893:13)
    at process.emit (node:events:518:28)
    at emitUnhandledRejection (node:internal/process/promises:252:13)
    at throwUnhandledRejectionsMode (node:internal/process/promises:388:19)
    at processPromiseRejections (node:internal/process/promises:475:17)
    at process.processTicksAndRejections (node:internal/process/task_queues:106:32)
If you keep encountering errors, please report to https://github.com/louislam/uptime-kuma/issues

What features or functionality does this pull request introduce or enhance?
This PR adds defensive callback validation in the deleteMonitor handler by checking if the callback exists before invoking it.

Btw, I'm skeptical that there's no issue created for this.

• Relates to feat: enhance monitor deletion functionality by adding child deletion… #6314 (not sure)

🛠️ Type of change

• 🐛 Bugfix (a non-breaking change that resolves an issue)
• ✨ New feature (a non-breaking change that adds new functionality)
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[Copilot]

Pull request overview

This PR fixes a crash in the deleteMonitor socket handler that occurs when the callback parameter is undefined. The error happens because the handler's backward compatibility logic for an optional middle parameter (deleteChildren) doesn't guarantee that a callback will always be defined. The fix adds defensive checks before invoking the callback.

Key changes:

• Added if (callback) validation before calling callback in success path
• Added if (callback) validation before calling callback in error path

[Copilot]

While the callback validation prevents the crash, this pattern is inconsistent with all other socket handlers in this file (e.g., pauseMonitor, resumeMonitor, deleteTag, deleteNotification) which always call callbacks without checking. The root issue is that the backward compatibility logic (lines 1059-1062) allows callback to be undefined when only monitorID is provided. Consider whether callbacks should be required for this handler (consistent with other handlers) or if the backward compatibility should explicitly handle the no-callback case with a default no-op function, e.g., callback = callback || (() => {}); after the backward compatibility check.