Improve error message when application/x-www-form-urlencoded is not used for token requests

Describe the feature

When a client sends an OAuth2 token request with an incorrect Content-Type header (for example, application/json instead of the required application/x-www-form-urlencoded), oauthlib currently responds with unsupported_grant_type.
This is misleading because the grant type itself is correct the issue is the request encoding.
The feature request is to return a more accurate and descriptive error message, such as:
"Content-Type must be application/x-www-form-urlencoded."
Additional context

This improvement would help developers quickly diagnose incorrect request formatting instead of searching for issues with their grant type.
The feature applies to OAuth2 and affects server-side processing of token endpoint requests.

Start at the server-side OAuth2 token endpoint processing described in the issue and reproduce a token request using application/json instead of application/x-www-form-urlencoded. The work is done when the incorrect encoding produces a clear Content-Type error while correctly encoded requests continue to process normally.