fix: fix OAuth flow request object handling

[clareliguori]

Motivation and Context

Fixes #1173

How Has This Been Tested?

New unit test fails before change and succeeds after

Example unit test failure before the change, highlighting the request handling bug:

       # Final request should be the original request with auth header
        final_request = await auth_flow.asend(token_response)
        assert final_request.headers["Authorization"] == "Bearer new_access_token"
        assert final_request.method == "GET"
>       assert str(final_request.url) == "https://api.example.com/mcp"
E       AssertionError: assert 'https://auth...zation-server' == 'https://api.example.com/mcp'
E         
E         - https://api.example.com/mcp
E         + https://auth.example.com/.well-known/oauth-authorization-server

tests/client/test_auth.py:581: AssertionError

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

Additional context