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_track_state_persistence.py

@@ -2,7 +2,7 @@ from datetime import datetime
 
 # Import the real models
 from src.mma import TrackState, Ticket
-from src.models import Metadata
+from src.type_aliases import Metadata
 # Import the persistence functions from project_manager
 from src.project_manager import save_track_state, load_track_state
 
@@ -66,4 +66,4 @@ def test_track_state_persistence(tmp_path) -> None:
   assert loaded_state.discussion[i]["content"] == original_state.discussion[i]["content"]
   assert loaded_state.discussion[i]["ts"] == original_state.discussion[i]["ts"]
   # Final check: deep equality of dataclasses
- assert loaded_state == original_state
+ assert loaded_state == original_state