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_rag_integration.py

@@ -7,7 +7,6 @@ import pytest
 from src.app_controller import AppController
 from src import ai_client
 from src import events
-from src import models
 from src.mcp_client import VectorStoreConfig, RAGConfig
 from src.result_types import Result
 
@@ -113,4 +112,4 @@ def test_rag_integration(mock_project):
    assert "Source: test_file.py" in sent_user_message
    
    # Verify that rag_engine.search was called with the original prompt
-   mock_rag_engine.search.assert_called_once_with("Tell me about the code.")
+   mock_rag_engine.search.assert_called_once_with("Tell me about the code.")