fix(ai): reset provider autocomplete state on search string change

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

src/command/cmd_ac.c

@@ -1697,6 +1697,7 @@ cmd_ac_reset(ProfWin* window)
     win_reset_search_attempts();
     win_close_reset_search_attempts();
     plugins_reset_autocomplete();
+    ai_providers_reset_ac();
 }
 
 void