fix(ai): BUG-044 capacités des modèles lues chez le fournisseur (Mistral vision)

Le panneau de modèle par défaut et la bulle ⓘ n'affichaient aucun modèle Mistral
« Vision capable » alors que GET api.mistral.ai/v1/models en déclare 28 : la table
de capacités était entièrement statique et aucun de ses motifs ne correspondait aux
familles Mistral actuelles (seul `pixtral`, retiré de l'API, les matchait).

- backend/provider_capabilities.py (nouveau) : capacités déclarées par le
  fournisseur (Mistral `capabilities`, OpenRouter `architecture`), détectées par
  la forme du payload, snapshot en cache process-wide (TTL 30 min, surchargeable
  par AI_CAPABILITIES_TTL_SECONDS) rempli par GET /api/config/ai-models.
- backend/model_capabilities.py : une déclaration prime sur la table statique
  pour chaque drapeau mentionné ; la table ne comble que le reste (Mistral ne
  déclare jamais `embedding`). Table corrigée pour le repli hors ligne : familles
  vision Mistral (ministral, magistral, mistral-small, mistral-medium,
  mistral-vibe-cli, labs-leanstral), mistral-ocr = vision sans chat, et défaut du
  fournisseur Mistral sans `embeddings` (mistral-large / codestral n'étaient plus
  des « embedders »).
- backend/ai_routes.py : GET /api/ai/model-capabilities reste sans appel réseau
  (cache froid → table statique).
- Tests : tests/test_provider_capabilities.py (nouveau), TestMistralFamilies et
  TestDeclaredCapabilities (bout en bout via l'API).
- Docs : CHANGELOG [Unreleased], registre + journal ISSUES_TODOLIST,
  fiche docs/features/ai-provider-picker.md (§L).

Vérifié : 28/28 modèles vision déclarés par Mistral détectés (0 avant), 0 écart
dans les deux sens ; pytest 1007 passed / 6 skipped ; ruff 0 (backend) ; mypy 0 ;
tests frontend unit 9/9 + IA 66/66 + validate-imports 37 modules ; instance de test
reconstruite et vérifiée sur http://localhost:2020.

tests/test_ai_models.py

@@ -14,6 +14,16 @@ from fastapi.testclient import TestClient
 
 from backend.main import _FALLBACK_MODELS, app
 
+#: Trimmed-down copy of what api.mistral.ai/v1/models really returns (BUG-044).
+MISTRAL_LIVE_PAYLOAD = {
+    "object": "list",
+    "data": [
+        {"id": "mistral-small-latest", "capabilities": {"completion_chat": True, "vision": True}},
+        {"id": "mistral-medium-latest", "capabilities": {"completion_chat": True, "vision": True}},
+        {"id": "mistral-embed", "capabilities": {"completion_chat": False, "vision": False}},
+    ],
+}
+
 
 @pytest.fixture
 def admin_client(tmp_path):
@@ -83,6 +93,16 @@ def _login_admin(client):
     return resp.json()["access_token"]
 
 
+@pytest.fixture(autouse=True)
+def _no_declared_caps():
+    """Provider declarations are cached process-wide: isolate every test."""
+    from backend.provider_capabilities import clear_declared_capabilities
+
+    clear_declared_capabilities()
+    yield
+    clear_declared_capabilities()
+
+
 # ── Unit tests for the fallback table itself ─────────────────────────────
 
 
@@ -339,3 +359,112 @@ class TestListModelsEndpoint:
         assert xiaomi_hdrs["api-key"] == "fake-key"
         # Bearer prefix must NOT be in the api-key value
         assert "Bearer" not in xiaomi_hdrs["api-key"]
+
+
+class TestDeclaredCapabilities:
+    """BUG-044 — the provider's own ``capabilities[]`` must reach the UI.
+
+    ``GET /api/config/ai-models`` is what fills the capability cache; the picker
+    bubble (``GET /api/ai/model-capabilities``) and the vision gate then read it.
+    Before the fix no Mistral model was ever reported as vision-capable.
+    """
+
+    def _serve_payload(self, monkeypatch, payload):
+        """Make the provider models call return ``payload`` verbatim."""
+        import json
+        import urllib.request as global_urllib_mod
+
+        body = json.dumps(payload).encode()
+
+        def fake_Request(url, *args, **kwargs):
+            class FakeResp:
+                def __enter__(self): return self
+                def __exit__(self, *a): pass
+                def read(self): return body
+
+            return FakeResp()
+
+        monkeypatch.setattr(global_urllib_mod, "Request", fake_Request, raising=True)
+        monkeypatch.setattr(
+            global_urllib_mod, "urlopen", lambda req, timeout=None: req, raising=True
+        )
+
+    def _fake_key(self, monkeypatch):
+        """main.py binds get_ai_key at import: patch that binding too."""
+        import backend.ai as aimod
+        import backend.main as bmain
+
+        monkeypatch.setattr(aimod, "get_ai_key", lambda name: "fake-mistral-key")
+        if hasattr(bmain, "get_ai_key"):
+            monkeypatch.setattr(bmain, "get_ai_key", lambda name: "fake-mistral-key")
+
+    def test_declared_vision_reaches_both_endpoints(self, admin_client, monkeypatch):
+        self._fake_key(monkeypatch)
+        self._serve_payload(monkeypatch, MISTRAL_LIVE_PAYLOAD)
+
+        token = _login_admin(admin_client)
+        headers = {"Authorization": f"Bearer {token}"}
+
+        listing = admin_client.get(
+            "/api/config/ai-models",
+            params={"provider": "mistral"},
+            headers=headers,
+        )
+        assert listing.status_code == 200, listing.text
+        data = listing.json()
+        assert data["source"] == "live"
+        assert data["capabilities"]["mistral-small-latest"]["vision"] is True
+        assert data["capabilities"]["mistral-medium-latest"]["vision"] is True
+        # The declaration also revokes: mistral-embed is not a chat model.
+        assert data["capabilities"]["mistral-embed"]["chat"] is False
+        assert data["capabilities"]["mistral-embed"]["embeddings"] is True
+
+        for model in ("mistral-small-latest", "mistral-medium-latest"):
+            detail = admin_client.get(
+                "/api/ai/model-capabilities",
+                params={"provider": "mistral", "model": model},
+                headers=headers,
+            )
+            assert detail.status_code == 200, detail.text
+            assert detail.json()["capabilities"]["vision"] is True, model
+
+    def test_text_only_mistral_models_stay_text_only(self, admin_client, monkeypatch):
+        """mistral-large-latest declares no vision in the real API — keep it so."""
+        self._fake_key(monkeypatch)
+        self._serve_payload(monkeypatch, MISTRAL_LIVE_PAYLOAD)
+
+        token = _login_admin(admin_client)
+        resp = admin_client.get(
+            "/api/config/ai-models",
+            params={"provider": "mistral"},
+            headers={"Authorization": f"Bearer {token}"},
+        )
+        assert resp.status_code == 200, resp.text
+        caps = resp.json()["capabilities"]["mistral-large-latest"]
+        assert caps["vision"] is False
+        assert caps["embeddings"] is False
+
+    def test_curated_fallback_covers_mistral_without_any_api_call(
+        self, admin_client, monkeypatch,
+    ):
+        """No API key: the curated list must still answer correctly (offline)."""
+        import backend.ai as aimod
+        import backend.main as bmain
+
+        monkeypatch.setattr(aimod, "get_ai_key", lambda name: None)
+        if hasattr(bmain, "get_ai_key"):
+            monkeypatch.setattr(bmain, "get_ai_key", lambda name: None)
+
+        token = _login_admin(admin_client)
+        resp = admin_client.get(
+            "/api/config/ai-models",
+            params={"provider": "mistral"},
+            headers={"Authorization": f"Bearer {token}"},
+        )
+        assert resp.status_code == 200, resp.text
+        data = resp.json()
+        assert data["source"] == "fallback"
+        caps = data["capabilities"]
+        assert caps["mistral-small-latest"]["vision"] is True
+        assert caps["mistral-large-latest"]["vision"] is False
+        assert caps["mistral-large-latest"]["embeddings"] is False