fix: Explicitly declare accpet type to json when exchanging oauth token

[JoJoJoJoJoJoJo]

When exchanging OAuth token, explicitly declare the accept type to JSON as it's potentially indicated

Motivation and Context

When using official github mcp server, the default response is not a valid json and causes json parse error. (Returned something like access_token=[some token]&scope=[some scope]&token_type=bearer). After adding the header the response is json. So the type may need to be explicitly declared if it will be treated as json format.

How Has This Been Tested?

Tested with github mcp server

Breaking Changes

No

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