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_model_capabilities.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import pytest
+
 from backend.model_capabilities import (
     CAPABILITY_KEYS,
     get_capabilities_for_models,
@@ -10,6 +12,16 @@ from backend.model_capabilities import (
 )
 
 
+@pytest.fixture(autouse=True)
+def _no_declared_caps():
+    """Curated-table tests must not see the process-wide declaration cache."""
+    from backend.provider_capabilities import clear_declared_capabilities
+
+    clear_declared_capabilities()
+    yield
+    clear_declared_capabilities()
+
+
 class TestCapabilityShape:
     def test_every_result_has_all_keys(self):
         for provider, model in [
@@ -84,3 +96,85 @@ class TestBatch:
         )
         assert result["qwen-max"]["vision"] is False
         assert result["qwen-vl-max"]["vision"] is True
+
+
+class TestMistralFamilies:
+    """BUG-044 — the curated fallback must know the current Mistral families.
+
+    This fallback is what answers when the provider declaration is unavailable
+    (no API key, offline, cold cache), so it has to agree with what
+    ``api.mistral.ai/v1/models`` declares today. Before the fix, no Mistral
+    model at all was reported as vision-capable — only the dead ``pixtral``
+    pattern matched.
+    """
+
+    @pytest.mark.parametrize(
+        "model",
+        [
+            "mistral-small-latest",
+            "mistral-small-2603",
+            "mistral-medium-latest",
+            "mistral-medium-3.5",
+            "ministral-3b-latest",
+            "ministral-8b-2512",
+            "ministral-14b-latest",
+            "magistral-small-latest",
+            "magistral-medium-latest",
+            "mistral-vibe-cli-latest",
+            "pixtral-12b-2409",
+        ],
+    )
+    def test_vision_chat_families(self, model):
+        caps = get_model_capabilities("mistral", model)
+        assert caps["vision"] is True, model
+        assert caps["chat"] is True, model
+
+    @pytest.mark.parametrize(
+        "model",
+        [
+            "mistral-large-latest",
+            "codestral-latest",
+            "open-mixtral-8x7b",
+            "voxtral-small-latest",
+            "mistral-code-latest",
+        ],
+    )
+    def test_text_only_models_are_not_vision(self, model):
+        caps = get_model_capabilities("mistral", model)
+        assert caps["vision"] is False, model
+        assert caps["chat"] is True, model
+
+    def test_ocr_models_are_vision_without_chat(self):
+        caps = get_model_capabilities("mistral", "mistral-ocr-latest")
+        assert caps["vision"] is True
+        assert caps["chat"] is False
+
+    def test_embeddings_only_for_the_embed_models(self):
+        assert get_model_capabilities("mistral", "mistral-embed")["embeddings"] is True
+        # Regression: every Mistral model used to inherit embeddings=True from
+        # the provider default, wrongly branding chat models as embedders.
+        assert get_model_capabilities("mistral", "mistral-large-latest")["embeddings"] is False
+        assert get_model_capabilities("mistral", "some-unknown-mistral")["embeddings"] is False
+
+    def test_current_api_vision_models_are_all_covered(self):
+        """Every model the API declares vision=true is vision-capable offline.
+
+        List captured from ``GET https://api.mistral.ai/v1/models`` (2026-09-15);
+        the live declaration layer covers renames, this guards the offline path.
+        """
+        api_vision_models = [
+            "magistral-medium-latest", "magistral-small-latest",
+            "ministral-14b-2512", "ministral-14b-latest",
+            "ministral-3b-2512", "ministral-3b-latest",
+            "ministral-8b-2512", "ministral-8b-latest",
+            "mistral-medium", "mistral-medium-2604", "mistral-medium-3",
+            "mistral-medium-3-5", "mistral-medium-3.5", "mistral-medium-latest",
+            "mistral-ocr-2512", "mistral-ocr-3", "mistral-ocr-3-0",
+            "mistral-ocr-4", "mistral-ocr-4-0", "mistral-ocr-4-1",
+            "mistral-ocr-latest",
+            "mistral-small-2603", "mistral-small-latest",
+            "mistral-vibe-cli-fast", "mistral-vibe-cli-latest",
+            "mistral-vibe-cli-with-tools",
+        ]
+        missing = [m for m in api_vision_models if not model_supports_vision("mistral", m)]
+        assert missing == [], f"curated fallback misses vision for: {missing}"