feat: Add webhook POST body validation

[No-Smoke]

Description

Adds validation to detect when webhook POST bodies may not be accessed correctly in downstream nodes.

Changes

• Added validateWebhookNodes() method to CreateWorkflowHandler
• Validates Code and Set nodes connected to webhook nodes
• Checks for proper body access patterns ($json.body, $input.first().json.body)
• Returns warnings in workflow creation response
• Non-breaking: only logs warnings, doesn't block creation

Validation Patterns

✅ $json.body.*
✅ $input.first().json.body.*
✅ $input.item.json.body.*
✅ {{ $json.body (template syntax)

Example Warning

⚠️  Webhook Validation Warnings:
   - Webhook node "webhook1" connects to Code node "processData" 
     that may not correctly access POST body. 
     Use $json.body.* or $input.first().json.body.* to access POST data.

Testing

Validated with workflows containing:

• Webhook → Code node (with/without proper body access)
• Webhook → Set node (with/without body references)
• Multiple webhook nodes in single workflow

Related Issues

Addresses common webhook POST body access issues that can lead to undefined data or workflow failures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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