refactor(ai_client): move PROVIDERS to src/ai_client.py; re-export via models.__getattr__

Phase 2 tasks 2.1 + 2.2 + 2.3a of the follow-up track.

PROVIDERS now lives in src/ai_client.py:56 (the canonical home for
AI-client-related constants per the HARD RULE on src/ files). The
list includes all 8 vendors: gemini, anthropic, gemini_cli,
deepseek, minimax, qwen, grok, llama.

Backward compat: src/models.py:PROVIDERS is exposed via a module-
level __getattr__ (PEP 562) that lazy-imports from src.ai_client.
The lazy approach was needed because src.ai_client imports
ToolPreset/BiasProfile/Tool from src.models at line 50, so a
top-level 'from src.ai_client import PROVIDERS' in models.py
would deadlock. Adding a branch to the existing __getattr__
in models.py (which also handles pydantic class factories) is
the surgical fix.

tests/test_provider_curation.py was stale (expected 5 providers
from before Qwen/Grok/Llama were added). Updated to 8.

New test: tests/test_providers_source_of_truth.py asserts:
- src.ai_client.PROVIDERS exists and matches the 8-provider list
- src.models.PROVIDERS still works (re-export)
- Both modules reference the SAME object (no drift)

Green confirmed: 4 provider tests pass.

tests/test_provider_curation.py

@@ -3,6 +3,6 @@ import src.app_controller
 
 def test_providers_moved_to_models():
  """Verify that PROVIDERS list is in models.py and removed from AppController."""
- expected_providers = ['gemini', 'anthropic', 'gemini_cli', 'deepseek', 'minimax']
+ expected_providers = ['gemini', 'anthropic', 'gemini_cli', 'deepseek', 'minimax', 'qwen', 'grok', 'llama']
  assert models.PROVIDERS == expected_providers
  assert not hasattr(src.app_controller.AppController, 'PROVIDERS')

tests/test_providers_source_of_truth.py

@@ -0,0 +1,23 @@
+"""Verify PROVIDERS is defined in src.ai_client (the source of truth)
+and re-exported from src.models (backward compat shim).
+
+Per the follow-up track's Naming Convention (HARD RULE), PROVIDERS
+lives in src/ai_client.py. src/models.py keeps a re-export
+shim so existing import sites don't break.
+"""
+from __future__ import annotations
+import src.models as models
+import src.ai_client as ai_client
+
+EXPECTED_PROVIDERS = ["gemini", "anthropic", "gemini_cli", "deepseek", "minimax", "qwen", "grok", "llama"]
+
+def test_providers_defined_in_src_ai_client() -> None:
+ assert hasattr(ai_client, "PROVIDERS")
+ assert ai_client.PROVIDERS == EXPECTED_PROVIDERS
+
+def test_providers_reexported_from_src_models() -> None:
+ assert hasattr(models, "PROVIDERS")
+ assert models.PROVIDERS == EXPECTED_PROVIDERS
+
+def test_providers_same_object_in_both_modules() -> None:
+ assert models.PROVIDERS is ai_client.PROVIDERS