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_deepseek_infra.py

@@ -42,7 +42,7 @@ def test_gui_providers_list() -> None:
   
       Check if 'deepseek' is in the GUI's provider list.
  """
- from src.models import PROVIDERS
+ from src.ai_client import PROVIDERS
  assert "deepseek" in PROVIDERS
 
 def test_deepseek_model_listing() -> None:
@@ -68,4 +68,4 @@ def test_gui_provider_list_via_hooks(live_gui: Any) -> None:
  # Attempt to set provider to deepseek to verify it's an allowed value
  client.set_value('current_provider', 'deepseek')
  time.sleep(0.5)
- assert client.get_value('current_provider') == 'deepseek'
+ assert client.get_value('current_provider') == 'deepseek'