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_project_serialization.py

@@ -3,7 +3,6 @@ import unittest
 import tempfile
 from pathlib import Path
 from src import project_manager
-from src import models
 from src.project_files import FileItem
 from src.app_controller import AppController
 
@@ -88,4 +87,4 @@ roles = ["User", "AI"]
   self.assertIn("Context", proj["discussion"]["roles"])
 
 if __name__ == "__main__":
- unittest.main()
+ unittest.main()