fix: only try to refresh access tokens if we have a refresh token or an expiry time

[vorporeal]

Motivation and Context

When connecting to the GitHub remote MCP server as an "OAuth App" client (as opposed to a "GitHub App" client), GitHub issues a persistent, non-expiring access token and no refresh token.

The current logic treats a lack of an expiry time as "the token is already expired", which is incorrect in this use case, as the token never expires, and we are unable to refresh it (as we were never issued a refresh token).

This fixes the issue by only attempting to refresh if we have either a refresh token or an expiry time.

How Has This Been Tested?

Tested it with a local build of Warp, and verified that instead of getting OAuth token refresh failed: No refresh token available errors, we now are able to properly make an authenticated connection to the GitHub remote MCP server.

Breaking Changes

No, this is not a breaking API change, and should not break any functional behavior.

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