fix(ai): use atomic operations for refcounting

Replace plain ref_count++/-- with g_atomic_int_inc and
g_atomic_int_dec_and_test for both AIProvider and AISession refcounts.
Prevents data races when ref/unref is called from worker threads.

src/ai/ai_client.c

@@ -192,7 +192,7 @@ static AIProvider*
 ai_provider_ref(AIProvider* provider)
 {
     if (provider) {
-        provider->ref_count++;
+        g_atomic_int_inc(&provider->ref_count);
     }
     return provider;
 }
@@ -203,9 +203,9 @@ ai_provider_unref(AIProvider* provider)
     if (!provider)
         return;
 
-    provider->ref_count--;
+    if (!g_atomic_int_dec_and_test(&provider->ref_count)) {
-    if (provider->ref_count > 0)
         return;
+    }
 
     g_free(provider->name);
     g_free(provider->api_url);
@@ -451,7 +451,7 @@ AISession*
 ai_session_ref(AISession* session)
 {
     if (session) {
-        session->ref_count++;
+        g_atomic_int_inc(&session->ref_count);
     }
     return session;
 }
@@ -462,9 +462,9 @@ ai_session_unref(AISession* session)
     if (!session)
         return;
 
-    session->ref_count--;
+    if (!g_atomic_int_dec_and_test(&session->ref_count)) {
-    if (session->ref_count > 0)
         return;
+    }
 
     g_free(session->provider_name);
     ai_provider_unref(session->provider);

src/ai/ai_client.h

@@ -22,7 +22,7 @@ typedef struct ai_provider_t
     gchar* org_id;     /* Optional organization ID */
     gchar* project_id; /* Optional project ID (for some providers) */
     GList* models;     /* List of available models (gchar*) */
-    guint ref_count;   /* Reference count */
+    gint32 ref_count;  /* Reference count (atomic) */
 } AIProvider;
 
 /**
@@ -35,7 +35,7 @@ typedef struct ai_session_t
     gchar* model;         /* Model identifier (e.g., "gpt-4", "sonar") */
     gchar* api_key;       /* API key for this session */
     GList* history;       /* Conversation history (GList of AIMessage*) */
-    guint ref_count;      /* Reference count */
+    gint32 ref_count;     /* Reference count (atomic) */
 } AISession;
 
 /* ========================================================================