test(ai): expand unit and functional coverage for /ai feature

Unit tests (tests/unittests/test_ai_client.c): +50 cases on top of the
existing 50 — total 100. Covers:
  - chat response parser (ai_parse_response): OpenAI content + Perplexity
    text formats, escape decoding, empty/null/missing inputs, format-string
    safety, multiline content
  - error envelope parser (ai_parse_error_message): standard envelope,
    nested escapes, missing fields, null/empty
  - extended JSON escape: \b, \f, \r, all-specials, UTF-8 pass-through
  - provider autocomplete cycling with >=2 matches and wrap-around
  - session edge cases: NULL args, 100-message order preservation,
    set_model(NULL), ref/unref(NULL)
  - provider edge cases: get/remove with NULL, double-remove, survival
    via session ref after ai_remove_provider
  - settings: multi-key independence, missing key, cross-provider
    isolation
  - model parsing edges: data not array, empty data, "id" outside data,
    multiple models
  - prefs round-trip: set token -> shutdown -> init -> token reloaded
    from disk (uses load_preferences fixture)

Functional tests:
  - Tier A (test_ai.c): 15 cases for the /ai command surface that don't
    need HTTP. Covers /ai help, providers list, set provider/token,
    start with/without key/unknown provider, clear, remove, default
    provider/model, switch without window, bad subcommand.
  - Tier B (test_ai_http.c + ai_http_stub.{c,h,py}): 5 cases that
    exercise the libcurl path against a local Python HTTP stub. The
    stub serves canned bodies in five modes (ok, openai, 401, 500,
    models) so each chat/models/error path is hit end-to-end without
    network.

Infrastructure:
  - TEST_GROUPS bumped to 5 in proftest.c; AI tests live in their own
    Group 5 because mixing them with stabber-driven tests in Group 4
    poisoned stbbr_stop() teardown.
  - PROF_FUNC_TEST_AI macro in functionaltests.c registers AI tests
    with ai_init_test() which wraps init_prof_test with a prof_connect()
    so stabber sees a graceful disconnect at teardown.
  - dist_check_DATA ships ai_http_stub.py with the source tarball.

Source-level changes to enable testing:
  - src/ai/ai_client.{c,h}: dropped 'static' from _parse_ai_response
    (renamed ai_parse_response) and _parse_error_response (renamed
    ai_parse_error_message); declared under a "Parsing helpers (exposed
    for testing)" section. Same approach as ai_parse_models_from_json.

Results in Docker (cproof-debian image):
  - 595/595 unit tests pass
  - 20/20 AI functional tests pass (Group 5)

tests/unittests/unittests.c

@@ -716,6 +716,65 @@ main(int argc, char* argv[])
         cmocka_unit_test(test_ai_json_escape_special_chars),
         cmocka_unit_test(test_ai_json_escape_percent_signs),
         cmocka_unit_test(test_ai_json_escape_backslash_quote),
+        /* Chat response parser */
+        cmocka_unit_test(test_ai_parse_response_openai_content),
+        cmocka_unit_test(test_ai_parse_response_perplexity_text),
+        cmocka_unit_test(test_ai_parse_response_text_preferred_over_content),
+        cmocka_unit_test(test_ai_parse_response_escaped_quote),
+        cmocka_unit_test(test_ai_parse_response_newline_escape),
+        cmocka_unit_test(test_ai_parse_response_empty_content),
+        cmocka_unit_test(test_ai_parse_response_missing_field),
+        cmocka_unit_test(test_ai_parse_response_null_input),
+        cmocka_unit_test(test_ai_parse_response_empty_input),
+        cmocka_unit_test(test_ai_parse_response_percent_signs_safe),
+        cmocka_unit_test(test_ai_parse_response_braces_in_content),
+        cmocka_unit_test(test_ai_parse_response_multiline_content),
+        /* Error response parser */
+        cmocka_unit_test(test_ai_parse_error_standard_envelope),
+        cmocka_unit_test(test_ai_parse_error_with_escapes),
+        cmocka_unit_test(test_ai_parse_error_no_error_field),
+        cmocka_unit_test(test_ai_parse_error_no_message_field),
+        cmocka_unit_test(test_ai_parse_error_null_input),
+        cmocka_unit_test(test_ai_parse_error_empty_input),
+        cmocka_unit_test(test_ai_parse_error_tab_escape),
+        cmocka_unit_test(test_ai_parse_error_backslash_escape),
+        /* Extended JSON escape coverage */
+        cmocka_unit_test(test_ai_json_escape_backspace),
+        cmocka_unit_test(test_ai_json_escape_formfeed),
+        cmocka_unit_test(test_ai_json_escape_carriage_return),
+        cmocka_unit_test(test_ai_json_escape_all_specials_combined),
+        cmocka_unit_test(test_ai_json_escape_passes_utf8_through),
+        /* Autocomplete cycling */
+        cmocka_unit_test_setup_teardown(test_ai_providers_find_cycles_through_many, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_providers_find_previous_walks_backwards, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_providers_find_wraps_around, ai_client_setup, ai_client_teardown),
+        /* Session edge cases */
+        cmocka_unit_test_setup_teardown(test_ai_session_add_message_null_session_no_crash, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_add_message_null_role_no_crash, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_add_message_null_content_no_crash, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_history_preserves_large_order, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_clear_empty_history, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_set_model_null_keeps_old, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_unref_null_no_crash, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_session_ref_null_returns_null, ai_client_setup, ai_client_teardown),
+        /* Provider edge cases */
+        cmocka_unit_test_setup_teardown(test_ai_get_provider_null_returns_null, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_remove_provider_null_returns_false, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_remove_provider_twice_second_fails, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_provider_survives_via_session_after_removal, ai_client_setup, ai_client_teardown),
+        /* Settings */
+        cmocka_unit_test_setup_teardown(test_ai_settings_multiple_keys_independent, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_settings_get_missing_returns_null, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_settings_isolated_between_providers, ai_client_setup, ai_client_teardown),
+        /* Model parsing edge cases */
+        cmocka_unit_test_setup_teardown(test_ai_parse_models_data_not_array, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_parse_models_empty_data_array, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_parse_models_id_outside_data_ignored, ai_client_setup, ai_client_teardown),
+        cmocka_unit_test_setup_teardown(test_ai_parse_models_multiple_models, ai_client_setup, ai_client_teardown),
+        /* Prefs round-trip (uses prefs+ai setup) */
+        cmocka_unit_test_setup_teardown(test_ai_prefs_round_trip_api_key, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
+        cmocka_unit_test_setup_teardown(test_ai_prefs_round_trip_remove_key, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
+        cmocka_unit_test_setup_teardown(test_ai_prefs_multiple_providers_persist, ai_client_setup_with_prefs, ai_client_teardown_with_prefs),
         // Flatfile export/import round-trip
         cmocka_unit_test(test_ff_roundtrip_simple_chat),
         cmocka_unit_test(test_ff_roundtrip_with_all_metadata),