feat(ai): implement robust JSON model parsing

Rewrote internal JSON parsing to correctly handle whitespace, escaped
quotes, and strict field context extraction. This prevents incorrect
field names or provider names from being added to the model list.

Added `ai_parse_models_from_json` public API to facilitate testing.

Changed `/ai models` default behavior to always fetch fresh models.
Replaced `--refresh` flag with `--cached` to display local cache.

Added comprehensive unit tests covering OpenAI and Perplexity formats,
array format, empty/null JSON, escaped quotes, and whitespace handling.

tests/unittests/test_ai_client.h

@@ -47,6 +47,14 @@ void test_ai_set_provider_setting(void** state);
 void test_ai_get_provider_setting(void** state);
 /* Model caching tests */
 void test_ai_models_are_fresh_initial(void** state);
+/* Model parsing tests */
+void test_ai_parse_models_openai_format(void** state);
+void test_ai_parse_models_perplexity_format(void** state);
+void test_ai_parse_models_array_format(void** state);
+void test_ai_parse_models_empty_json(void** state);
+void test_ai_parse_models_null_handling(void** state);
+void test_ai_parse_models_escaped_quotes(void** state);
+void test_ai_parse_models_with_whitespace(void** state);
 /* AI autocomplete integration tests */
 void test_ai_start_provider_autocomplete_only_on_exact(void** state);
 void test_ai_models_find_null_session(void** state);