When typing /ai start <prefix><tab>, the autocomplete was not properly
handling search string changes. For example, typing "/ai start o<tab>"
would return "openai", but then typing "/ai start p<tab>" would still
return "openai" because the autocomplete state was not reset.
This commit:
- Adds ai_providers_reset_ac() function to reset provider autocomplete
state, following the same pattern as accounts_reset_all_search()
- Integrates ai_providers_reset_ac() into cmd_ac_reset() for proper
state cleanup when user presses Ctrl+C or switches contexts