Management API: Content-Type header not enforced - text/plain accepted

Bug Description

The Management API accepts requests with Content-Type: text/plain when it should only accept application/json.

Steps to Reproduce

curl -X POST "https://api-dev.invioinsights.com/admin/v1/products" \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: text/plain" \
  -d '{"productId": "TEST-1", "name": "Test"}'

Expected Behavior

Return 415 Unsupported Media Type or 400 Bad Request when Content-Type is not application/json

Actual Behavior

Request is processed successfully (HTTP 201)

Impact

• Security: Potential bypass of content type security controls
• Consistency: API should be strict about accepted content types

Test Details

• Test #1 from QA batch
• Environment: invio-dev