Don't throw errors when Discord channel is not found

[garrrikkotua]

Changes proposed ✍️

What

🤖 Generated by Copilot at 5554438

This pull request fixes a bug in the Discord integration that caused errors when fetching deleted or inaccessible messages during webhook processing. It adds error handling and null checks to getMessage.ts and processWebhookStream.ts to gracefully skip such messages.
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🤖 Generated by Copilot at 5554438

Unknown Channel error
Discord message may be null
Skip it in the fall

Why

How

🤖 Generated by Copilot at 5554438

• Handle Unknown Channel error when fetching Discord messages (link)
• Skip parsing null messages in webhook processing (link)

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.
• Add screehshots to the PR description for relevant FE changes
• New backend functionality has been unit-tested.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.