Test a dotAI provider connection

Builds the provider client for the given capability from the posted configuration and issues one minimal real request against the provider (a short chat reply, a one-line embedding, or a single test image). Masked credential fields ("*****") in the posted config are resolved against the real value already stored for siteId before testing. Returns success=false with a message on any validation or provider error rather than an HTTP error status, so the caller can always render the result.