AI provider loading: timeouts and empty states

The AI provider list (Pro) can no longer hang forever or stick on "Testing…":

• 30-second client-side ceiling — if the provider call hasn't answered in 30 seconds (hung connection, stalled worker), it aborts with a plain-English timeout message instead of waiting on the browser to give up.
• Proper empty state — when the dashboard legitimately returns zero providers, the editor says "no providers came back" rather than sitting on "Testing…" as if still working.

Joomla 3

Identical on the Joomla 3 edition (v2.3.0).