Duplicate AuthTest API call on every startup

[mark-liu]

Each startup path (newWithXOXP, newWithXOXC) calls AuthTest() twice:

1. validateAuthAndGetTeamID() — calls AuthTest to get the TeamID for cache paths
2. NewMCPSlackClient() — calls AuthTest again to configure the edge client

The first response is discarded after extracting TeamID. The second rebuilds the same data.

When running multiple instances (e.g. separate workspaces), this doubles the auth API calls at startup for no reason, increasing the chance of hitting Slack rate limits.

Fix: return the full AuthTestResponse from validateAuthAndGetTeamID and pass it into NewMCPSlackClient so it can skip the redundant call.

I have a fix ready — will open a PR together with #254.