fix(uptimekuma): fix critical bugs in pull mode and webhook handling

[ART-R2D275]

Summary

Fix critical bugs in UptimeKuma provider for both pull mode and webhook handling.

Fixes #5655

Changes

Bug Fixes

• Fix heartbeats.append() outside for-loop — Only last monitor was being reported instead of all monitors
• Fix _format_datetime string concatenation — Was causing TypeError
• Fix webhook _format_alert referencing non-existent fields — Was causing KeyError on every webhook

Improvements

• Add AlertSeverity mapping from Uptime Kuma status codes
• Add FINGERPRINT_FIELDS for proper alert deduplication
• Add try/finally for connection cleanup
• Add bulk monitor fetching for better performance
• Add STATUS_MAP with both numeric and string key support

Testing

• Tested pull mode with multiple monitors
• Tested webhook payload handling
• Verified connection cleanup

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[greptile-apps]

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[ART-R2D275]

Hi maintainers 👋

The CLA assistant bot is requesting a signature, but when I try to sign at https://cla-assistant.io/keephq/keep it returns:

"There is no CLA to sign for keephq/keep (Not Found)"

Could you help resolve this? Happy to sign if there's a working link.

Thanks!