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/ai/ai_client.c

@@ -395,22 +395,15 @@ ai_list_providers(void)
  * Provider autocomplete state
  * ======================================================================== */
 
-/* Stateful provider name finder with case-sensitive matching.
- * Maintains position in sorted list for deterministic tab-completion cycling.
- * The autocomplete is kept in sync via ai_add_provider/ai_remove_provider. */
 gchar*
 ai_providers_find(const char* const search_str, gboolean previous, void* context)
 {
-    /* Initialize autocomplete on first use */
     if (!providers_ac) {
-        providers_ac = autocomplete_new();
+        log_debug("[AI-AC] Uninitialized call to ai_providers_find");
+        return NULL;
     }
 
-    /* NULL search_str is treated as empty string for cycling */
-    const char* effective_search = (search_str != NULL) ? search_str : "";
-
-    /* Use stateful autocomplete */
-    return autocomplete_complete(providers_ac, effective_search, FALSE, previous);
+    return autocomplete_complete(providers_ac, search_str, FALSE, previous);
 }
 
 /* Stateful model name finder for autocomplete.
@@ -444,6 +437,17 @@ ai_models_find(const char* const search_str, gboolean previous, void* context)
     return result;
 }
 
+/* Reset the provider autocomplete state.
+ * Called from cmd_ac_reset() to clear autocomplete state when the user presses Ctrl+C. */
+void
+ai_providers_reset_ac(void)
+{
+    if (!providers_ac) {
+        return;
+    }
+    autocomplete_reset(providers_ac);
+}
+
 gchar*
 ai_get_provider_key(const gchar* provider_name)
 {