Url encoded token response

[sebastian-velten]

Parsing the token response now depends on the received Content-Type.
This allows the MCP Client to deal with a application/x-www-form-urlencoded token response.

Motivation and Context

I played around with some example OAuth secured MCP servers and I noticed, that not all authorization servers are willing to return the token response as application/json, for instance the Github MCP Server auth server. As a a consequence the auth flow crashed at this point, because the response could not be parsed as OAuthToken from JSON.

How Has This Been Tested?

I ran an MCP Client (also built with this Python SDK) and tried to connect to OAuth secured MCP Servers.
When I tested the Github MCP Server, I was unhappy to see the auth flow crashes. With my suggested code changes, I tested again and authorization went fine this time.

Breaking Changes

No code or configurations need to be updated.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context