test(ai): align parse_response tests with chat completions API

The chat-completions refactor removed the legacy Perplexity "text"
extraction but left two tests asserting the old behavior, breaking
make check on every CI flavor:

- test_ai_parse_response_perplexity_text expected the legacy /v1/agent
  nested format to parse; it now yields NULL — renamed to
  test_ai_parse_response_legacy_text_format_unsupported.
- test_ai_parse_response_text_preferred_over_content expected "text"
  to win; "content" is authoritative now — renamed to
  test_ai_parse_response_content_preferred_over_text.

Add test_ai_set_provider_setting_reserved_key covering the new
reserved-key rejection.

src/ai/ai_client.h

@@ -17,15 +17,15 @@ typedef struct ai_message_t
  */
 typedef struct ai_provider_t
 {
-    gchar* name;           /* Provider name (e.g., "openai", "perplexity") */
+    gchar* name;                   /* Provider name (e.g., "openai", "perplexity") */
-    gchar* api_url;        /* API endpoint URL */
+    gchar* api_url;                /* API endpoint URL */
-    gchar* project_id;     /* Optional project ID (for some providers) */
+    gchar* project_id;             /* Optional project ID (for some providers) */
-    gchar* default_model;  /* Default model for this provider */
+    gchar* default_model;          /* Default model for this provider */
     pthread_mutex_t settings_lock; /* Protects settings (iterated by the request thread) */
-    GHashTable* settings;  /* Extensible per-provider settings (e.g., temperature=0.7) */
+    GHashTable* settings;          /* Extensible per-provider settings (e.g., temperature=0.7) */
-    GList* models;         /* Cached models (gchar*) */
+    GList* models;                 /* Cached models (gchar*) */
-    gboolean models_fresh; /* Whether models cache is current */
+    gboolean models_fresh;         /* Whether models cache is current */
-    gint32 ref_count;      /* Reference count (atomic) */
+    gint32 ref_count;              /* Reference count (atomic) */
 } AIProvider;
 
 /**