docs: Document tag replacement behaviour for Zendesk updates

[jabbson]

Added warning about tag replacement behaviour when updating tickets in Zendesk.

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Note

Add a warning in Zendesk node docs about tag replacement during ticket updates and how to safely add tags without overwriting.

• Docs:
  • Zendesk node (docs/integrations/builtin/app-nodes/n8n-nodes-base.zendesk.md):
    • Add warning about tag replacement when using Update Ticket with Tag Names or IDs.
    • Provide guidance to preserve existing tags (merge current tags, use additional_tags, or /tags endpoint) with links to Zendesk docs.

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