Fix /.well-known/oauth-authorization-server dropping path

[calclavia]

This fixes the TS SDK so it properly handles the provided path in the authorizationServerUrl, as per rfc 8414:
https://datatracker.ietf.org/doc/html/rfc8414#section-3

Authorization servers supporting metadata MUST make a JSON document containing metadata as specified in Section 2 available at a path formed by inserting a well-known URI string into the authorization server's issuer identifier between the host component and the path component, if any. By default, the well-known URI string used is "/.well-known/oauth-authorization-server". This path MUST use the "https" scheme. The syntax and semantics of ".well-known" are defined in RFC 5785 [RFC5785]. The well-known URI suffix used MUST be registered in the IANA "Well-Known URIs" registry [IANA.well-known].

Motivation and Context

Without this change, auth servers hosted at a subpath will break.

How Has This Been Tested?

Yes, this is being used at Smithery to handle servers hosted under different subpaths.

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

Relevant reviewer: @ihrpr

[ihrpr]

Thank you!

[ihrpr]

added fallback for backwards compatibility in #692