fix(agent): purge les api_base LLM obsoletes (Mistral/Cohere /v2)

Un api_base stocke (ex. https://api.mistral.ai/v2) ecrasait l'URL par defaut corrigee et faisait echouer le test de connexion. Desormais: normalisation a l'ecriture (une base egale au defaut n'est pas stockee), possibilite de vider le champ (api_base='' vs None), et migration 14 qui efface les bases obsoletes/redondantes dans user_llm_keys et llm_config.

Tests de non-regression ajoutes.

app/services/llm_config.py

@@ -101,8 +101,12 @@ def set_llm_config(*, provider: str | None = None, model: str | None = None,
             cfg["verified"] = 0
             cfg["verified_model"] = ""
             cfg["last_error"] = ""
-    if api_base is not None and api_base.strip():
-        cfg["api_base"] = api_base.strip()
+    if api_base is not None:
+        # Normalise so a base equal to the provider default is stored as "use
+        # default" — a later correction of PROVIDERS then applies automatically.
+        cfg["api_base"] = _normalize_api_base(
+            provider or cfg.get("provider") or "offline", api_base
+        )
     if clear_keys:
         cfg["api_key"] = ""
         cfg["api_base"] = ""
@@ -222,18 +226,44 @@ def list_user_llm_keys(user_id: int) -> list[dict]:
     return [_mask_key(r) for r in rows]
 
 
+def _default_api_base(provider: str) -> str:
+    """The OpenAI-compatible base URL the app uses by default for a provider."""
+    base = (PROVIDERS.get(provider.lower()) or (None, None))[0]
+    return (base or "").rstrip("/")
+
+
+def _normalize_api_base(provider: str, api_base: str) -> str:
+    """Drop an api_base that just repeats the provider default.
+
+    Storing the default as a per-user override freezes it: a later correction
+    of the provider URL (e.g. Cohere `/v2` → `/compatibility/v1`) would never
+    apply. An empty value means "use the provider default".
+    """
+    value = (api_base or "").strip()
+    if value.rstrip("/") == _default_api_base(provider):
+        return ""
+    return value
+
+
 def upsert_user_llm_key(user_id: int, provider: str, *, api_key: str = "",
-                        api_base: str = "", default_model: str = "",
+                        api_base: str | None = None, default_model: str = "",
                         models: list[str] | None = None) -> dict:
     """Upsert a user's provider key. Empty api_key keeps the existing one
     (allows saving model/base without re-typing the key). Saving a *different*
-    key resets the `verified` flag so the provider must pass a test again."""
+    key resets the `verified` flag so the provider must pass a test again.
+
+    ``api_base`` uses ``None`` to mean "keep the stored value" and an empty
+    string to explicitly reset it to the provider default.
+    """
     from app.db import get_conn
 
     provider = provider.lower()
     existing = get_user_llm_key(user_id, provider)
     new_key = api_key if api_key else (existing.get("api_key", "") if existing else "")
-    new_base = api_base if api_base else (existing.get("api_base", "") if existing else "")
+    if api_base is None:
+        new_base = (existing.get("api_base", "") if existing else "")
+    else:
+        new_base = _normalize_api_base(provider, api_base)
     new_model = default_model if default_model else (existing.get("default_model", "") if existing else "")
     new_models = models if models is not None else (
         json.loads(existing["models_json"]) if existing and existing.get("models_json") else []