OAuth Flow not working for Atlassian MCP and Github MCP

Describe the bug
The OAuth flow does not work for both Atlassian MCP and Github MCP when using the C# SDK. After initiating the authentication process, the redirect or token exchange does not complete successfully, resulting in authentication failure. It throws an error:

throw new McpException($"Failed to find .well-known/openid-configuration or .well-known/oauth-authorization-server metadata for authorization server: '{authServerUri}'");

Error Message is as follows:
ModelContextProtocol.McpException: 'Failed to find .well-known/openid-configuration or .well-known/oauth-authorization-server metadata for authorization server: 'https://github.com/login/oauth/''

To Reproduce
Steps to reproduce the behavior:

1. Clone the csharp-sdk Repo
2. Execute the "ProtectedClient" Repo and set the serverurl to var serverUrl = "https://api.githubcopilot.com/mcp/";
3. Error is thrown, no OAuth flow gets initiated

Expected behavior
I expected it to launch an authorization URL that I can use to authenticate against Github or Atlassian, just like it works with FastMCP.

Start with the ProtectedClient sample and its serverUrl set to https://api.githubcopilot.com/mcp/. Trace the OAuth metadata lookup that reports the missing .well-known/openid-configuration or .well-known/oauth-authorization-server metadata, then compare the GitHub and Atlassian flows. Done means the sample launches an authorization URL and completes authentication for the affected MCP servers.