feat: add Sematext Cloud provider (webhook integration)

[cairon-ab]

Summary

Adds a Sematext Cloud provider that receives alerts via Sematext's Custom Notification Hook (webhook).

Closes #607

What's included

• Provider implementation (keep/providers/sematext_provider/sematext_provider.py):

  • Webhook-only provider — no API credentials required on the Keep side
  • Maps all Sematext alert rule types (heartbeat, threshold, anomaly, logs, RUM, synthetics)
  • Handles backToNormal for automatic resolution
  • Severity mapping from Sematext priority field (string or int)
  • Group-by filter values propagated as labels
  • Stable fingerprint generation based on applicationId, ruleType, title, and filter values

• Tests (tests/sematext_provider/test_sematext_provider.py):

  • Unit tests for all helper functions
  • _format_alert tests covering firing, resolved, severity mapping, edge cases
  • Tests for group-by filter handling, minimal payloads, all rule types

• Documentation (docs/providers/documentation/sematext-provider.mdx):

  • Step-by-step webhook setup instructions
  • Supported alert types table
  • Status and severity mapping reference

Sematext webhook payload

The provider expects the standard Sematext Custom Notification Hook format:

{
  "backToNormal": "$backToNormal",
  "ruleType": "$ruleType",
  "description": "$description",
  "title": "$title",
  "applicationId": "$applicationId",
  "url": "$url",
  "createTimestamp": "$createTimestamp",
  "troubleshootUrl": "$troubleshootUrl"
}

Docs: https://sematext.com/docs/integration/alerts-webhooks-integration/

Signed-off-by: Cairon cairon-ab@users.noreply.github.com

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