fix(ai): add mutex to AISession for thread safety

Introduce a pthread mutex to protect all AISession fields from
concurrent access between the main thread and background request
worker.

- Initialize and destroy mutex during session lifecycle
- Lock session state when modifying or reading history, model, and
  provider fields
- Snapshot session state under lock in the request worker thread to
  prevent UAF races during API calls
- Refactor _build_json_payload to accept direct parameters for safe
  usage under lock
- Update command handlers to safely switch provider/model under lock

src/ai/ai_client.h

@@ -32,6 +32,7 @@ typedef struct ai_provider_t
  */
 typedef struct ai_session_t
 {
+    pthread_mutex_t lock; /* Protects all session fields below */
     gchar* provider_name; /* Provider name */
     AIProvider* provider; /* Provider configuration */
     gchar* model;         /* Model identifier (e.g., "gpt-4", "sonar") */
@@ -238,6 +239,45 @@ const gchar* ai_session_get_model(AISession* session);
  */
 void ai_session_set_model(AISession* session, const gchar* model);
 
+/**
+ * Atomically switch session provider, model, and API key.
+ * All mutations happen under the session lock to prevent races with
+ * _ai_request_thread() which snapshots session state before making requests.
+ *
+ * @param session The session
+ * @param provider_name New provider name
+ * @param model New model identifier
+ * @param api_key New API key (caller must free after calling this)
+ */
+void ai_session_switch(AISession* session, const gchar* provider_name,
+                       const gchar* model, gchar* api_key);
+
+/**
+ * Atomically switch session provider, model, and API key.
+ * All mutations happen under the session lock to prevent races with
+ * _ai_request_thread() which snapshots session state before making requests.
+ *
+ * @param session The session
+ * @param provider_name New provider name
+ * @param model New model identifier
+ * @param api_key New API key (caller must free after calling this)
+ */
+void ai_session_switch(AISession* session, const gchar* provider_name,
+                       const gchar* model, gchar* api_key);
+
+/**
+ * Atomically switch session provider, model, and API key.
+ * All mutations happen under the session lock to prevent races with
+ * _ai_request_thread() which snapshots session state before making requests.
+ *
+ * @param session The session
+ * @param provider_name New provider name
+ * @param model New model identifier
+ * @param api_key New API key (caller must free after calling this)
+ */
+void ai_session_switch(AISession* session, const gchar* provider_name,
+                       const gchar* model, gchar* api_key);
+
 /* ========================================================================
  * Request Handling
  * ======================================================================== */