fix(mcp): send custom headers regardless of OAuth settings - #5273

Previously, custom headers (e.g., Bearer tokens) were only sent when
oauth was explicitly set to false. This caused a regression where
configs like:

  {
    "type": "remote",
    "url": "...",
    "headers": { "Authorization": "Bearer ..." }
  }

would fail because headers weren't being sent on the initial request.

Now headers are always sent when provided, regardless of OAuth settings.