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.

tests/test_agent.py

@@ -106,6 +106,50 @@ def test_llm_client_openai_compatible_bases(client):
     assert PROVIDERS["sensenova"][0].endswith("/compatible-mode/v1")
 
 
+def test_user_llm_key_api_base_normalization(client):
+    """A stored api_base equal to the provider default is not persisted as an
+    override, so a corrected provider URL always takes effect."""
+    from app.services.llm_config import get_user_llm_key, upsert_user_llm_key
+
+    uid = _admin_id()
+    # default base → stored as empty (no override)
+    upsert_user_llm_key(uid, "mistral", api_key="sk-test",
+                        api_base="https://api.mistral.ai/v1")
+    assert get_user_llm_key(uid, "mistral")["api_base"] == ""
+    # custom base → kept
+    upsert_user_llm_key(uid, "mistral", api_base="https://proxy.example.com/v1")
+    assert get_user_llm_key(uid, "mistral")["api_base"] == "https://proxy.example.com/v1"
+    # api_base="" resets the override, api_base=None keeps it
+    upsert_user_llm_key(uid, "mistral", api_base="")
+    assert get_user_llm_key(uid, "mistral")["api_base"] == ""
+    upsert_user_llm_key(uid, "mistral", api_base="https://proxy.example.com/v1")
+    upsert_user_llm_key(uid, "mistral", default_model="mistral-large-latest")
+    assert get_user_llm_key(uid, "mistral")["api_base"] == "https://proxy.example.com/v1"
+
+
+def test_migration_clears_stale_llm_api_bases(client):
+    """Migration 14 clears legacy/incorrect api_base values so the corrected
+    provider default applies (e.g. Mistral `/v2` → `/v1`)."""
+    from app.db import get_conn
+    from app.migrations import _migration_fix_llm_api_bases
+    from app.services.llm_config import get_user_llm_key, upsert_user_llm_key
+
+    uid = _admin_id()
+    upsert_user_llm_key(uid, "mistral", api_key="sk-test")
+    with get_conn() as conn:
+        conn.execute(
+            "UPDATE user_llm_keys SET api_base='https://api.mistral.ai/v2' "
+            "WHERE user_id=? AND provider='mistral'",
+            (uid,),
+        )
+        conn.commit()
+    assert get_user_llm_key(uid, "mistral")["api_base"] == "https://api.mistral.ai/v2"
+    with get_conn() as conn:
+        _migration_fix_llm_api_bases(conn)
+        conn.commit()
+    assert get_user_llm_key(uid, "mistral")["api_base"] == ""
+
+
 def test_llm_client_offline_no_credentials(client):
     from app.services.llm_client import LLMClient
     llm = LLMClient(provider="openai", api_key="")