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/migrations.py

@@ -547,3 +547,44 @@ def _migration_v511_wiki_v512_templates(conn: sqlite3.Connection) -> None:
     conn.execute(
         "CREATE INDEX IF NOT EXISTS idx_pgt_creator ON page_global_templates(created_by)"
     )
+
+
+@register(14, "v5.13.0: fix stale LLM provider api_base values")
+def _migration_fix_llm_api_bases(conn: sqlite3.Connection) -> None:
+    """Clear `api_base` values that freeze a provider URL to a wrong/default value.
+
+    The Agent "Test connection" uses the per-user (or global) stored `api_base`
+    when present, so an old/incorrect value (e.g. Mistral `…/v2`, Cohere
+    `…/v2`, Google native `/v1beta`) keeps failing even after `PROVIDERS` is
+    corrected. Two kinds of rows are reset to the provider default:
+
+      * known-wrong legacy bases from earlier releases;
+      * a stored base identical to the current provider default (a no-op
+        override that would block future default changes).
+    """
+    from app.services.llm_client import PROVIDERS
+
+    legacy: dict[str, set[str]] = {
+        "mistral": {"https://api.mistral.ai/v2"},
+        "cohere": {"https://api.cohere.com/v2", "https://api.cohere.com/v1",
+                   "https://api.cohere.ai/v2"},
+        "google": {"https://generativelanguage.googleapis.com/v1beta"},
+        "perplexity": {"https://api.perplexity.ai/v1"},
+        "chutes": {"https://api.chutes.ai/v1"},
+        "sensenova": {"https://token.sensenova.cn/v1"},
+        "ltx": {"https://api.ltx.io/v1"},
+        "memtensor": {"https://memos.memtensor.cn/api/openmem/v1"},
+    }
+    for provider, (base, _) in PROVIDERS.items():
+        if base:
+            legacy.setdefault(provider, set()).update({base, base.rstrip("/")})
+
+    for provider, bases in legacy.items():
+        variants = {b for b in bases if b}
+        variants |= {b.rstrip("/") for b in bases if b}
+        for table in ("user_llm_keys", "llm_config"):
+            for value in variants:
+                conn.execute(
+                    f"UPDATE {table} SET api_base='' WHERE provider=? AND api_base=?",
+                    (provider, value),
+                )