fix(auth): preserve OAuth discovery transport errors

[tjni]

Motivation and Context

OAuth discovery currently treats network and server failures as missing metadata, which can incorrectly make a server appear not to support OAuth even when the root cause could be a client-side issue or a server-side server error.

This change propagates such errors to clients so that they can be displayed. I think it aligns more with how this SDK in other languages (I checked Go) behave.

How Has This Been Tested?

Currently, in Codex, adding Linear MCP server in a node:22-bookworm-slim container without ca-certificates installed shows the Linear MCP server as being unavailable even though the root cause is missing CA certificates on the system. With this change, the error is now clearer (even though Codex needs some change too in order to surface it properly).

Breaking Changes

N/A

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