fix: parse raw body Buffer in Vercel webhook controller

[ThyMinimalDev]

What does this PR do?

Fixes a bug where the Vercel webhook endpoint was returning 200 OK but not actually triggering the trigger.dev promotion.

Root cause: The RawBodyMiddleware provides the request body as a Buffer for signature verification. However, the controller was trying to access payload.type directly on the Buffer, which is always undefined. This caused the check payload.type !== "deployment.promoted" to always be true, returning early with { status: "ignored" } without calling the trigger.dev API.

Fix: The controller now properly parses the raw Buffer into JSON before checking the event type. It also handles the test environment case where the body may already be a parsed object.

• Follow-up to PR feat: add webhook endpoint for Vercel deployment promoted event to promote trigger.dev #27340

How should this be tested?

1. Deploy to Vercel and trigger a deployment promotion webhook
2. Verify that logs now show "Vercel Promoted! Promoting Trigger.dev to: {version}"
3. Verify that the trigger.dev API is actually called

Environment variables required:

• VERCEL_PROMOTE_WEBHOOK_SECRET - webhook signature verification
• TRIGGER_SECRET_KEY - bearer token for trigger.dev API
• TRIGGER_VERSION - deployment version to promote

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - no docs changes needed.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

Human Review Checklist

• Verify the JSON parse error handling is appropriate (returns error status instead of throwing)
• Confirm the Buffer detection works correctly in production with RawBodyMiddleware

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Link to Devin run

https://app.devin.ai/sessions/0531a7a6f6d6442e92e73eb9accc9adc

Requested by

@ThyMinimalDev

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