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.

tests/unittests/test_ai_client.h

@@ -43,6 +43,7 @@ void test_ai_providers_find_case_insensitive(void** state);
 void test_ai_set_provider_default_model(void** state);
 void test_ai_get_provider_default_model(void** state);
 void test_ai_set_provider_setting(void** state);
+void test_ai_set_provider_setting_reserved_key(void** state);
 void test_ai_get_provider_setting(void** state);
 /* Model caching tests */
 void test_ai_models_are_fresh_initial(void** state);
@@ -64,8 +65,8 @@ int ai_client_teardown_with_prefs(void** state);
 
 /* Chat response parser tests (ai_parse_response) */
 void test_ai_parse_response_openai_content(void** state);
-void test_ai_parse_response_perplexity_text(void** state);
-void test_ai_parse_response_text_preferred_over_content(void** state);
+void test_ai_parse_response_legacy_text_format_unsupported(void** state);
+void test_ai_parse_response_content_preferred_over_text(void** state);
 void test_ai_parse_response_escaped_quote(void** state);
 void test_ai_parse_response_newline_escape(void** state);
 void test_ai_parse_response_empty_content(void** state);