test: migrate src.models bare imports + PROVIDERS/Metadata sub-imports to direct subsystems

Removed 18 unused 'from src import models' imports. Migrated the
explicit sub-imports:
- 'from src.models import PROVIDERS' -> 'from src.ai_client import PROVIDERS'
  (test_minimax_provider x2, test_deepseek_infra)
- 'from src.models import Metadata' -> 'from src.type_aliases import Metadata'
  (test_project_manager_tracks, test_track_state_persistence, test_track_state_schema)

Kept the 2 'import src.models as models' in test_provider_curation +
test_providers_source_of_truth (they verify the backward-compat shim
still re-exports PROVIDERS) and the 2 self-tests
(test_models_no_top_level_pydantic + test_models_no_top_level_tomli_w)
that exercise the shim's no-leak invariants.

tests/test_gui_kill_button.py

@@ -1,6 +1,5 @@
 import pytest
 from unittest.mock import MagicMock, patch
-from src import models
 
 from src.mma import Ticket
 def test_gui_has_kill_button_method():
@@ -46,4 +45,4 @@ def test_render_ticket_queue_table_columns():
   app._cb_kill_ticket = MagicMock()
   render_ticket_queue(app)
   columns_called = [call[0][0] for call in mock_imgui.table_setup_column.call_args_list]
-  assert "Actions" in columns_called, f"Expected Actions column, got: {columns_called}"
+  assert "Actions" in columns_called, f"Expected Actions column, got: {columns_called}"