fix: token refresh error mutation breaks instanceof FgaApiAuthenticationError

[aaguiarz]

Summary

When client-credentials token refresh fails with a non-auth API error (e.g. 500), Credentials.refreshAccessToken() mutates an existing FgaApiError instance by assigning constructor, name, and auth fields.

Impact

Mutating object properties does not change the prototype chain, so instanceof FgaApiAuthenticationError fails even though the error is intended to be treated as authentication failure.

Expected

Token-refresh failures should throw a real FgaApiAuthenticationError instance with auth context.

Proposed Fix

Construct and throw a new FgaApiAuthenticationError (or equivalent wrapper) instead of mutating an existing error object.