Parking the SDK/marketplace bit, that was me jumping ahead. What I see as Concrete today (imho):
ValidateConfig is the same skeleton in dropbox / onedrive / notion / jira / github (parse + null check + return).
TestConnection has the same try/catch shape but error formatting already drifts (extractErrorMessage in dropbox/onedrive/notion vs inline instanceof Error in jira/github). Slack diverged into per-field errors, which is the skeleton hitting its ceiling.
Proposing a small PR: BaseConnector.validateConfigWithSchema(schema, label) + BaseConnector.runConnectionTest({ label, probe }), migrating dropbox/onedrive/notion as proof. If it holds across the next 2-3 connectors, we can revisit whether the surface deserves a stable package later. wdyt?