fix(models): reassign default endpoint when current default is disabled (#3649)

Adding a new endpoint only auto-set the global default chat endpoint when
none was configured (`if not settings.get("default_endpoint_id")`). When the
existing default pointed at an endpoint the user had since disabled, it was
never reassigned, so features that read the raw `default_endpoint_id` setting
(notably Memory → Tidy) failed with "No default model configured — set one in
Settings" even though an enabled endpoint existed.

Reassign the default when the configured endpoint is missing/disabled, via a
new pure `_default_endpoint_needs_assignment` helper. Adds unit coverage for
the helper plus route-level regression tests for the disabled/enabled cases.

Fixes #3586

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

routes/model_routes.py

@@ -123,6 +123,21 @@ def _clear_user_pref_endpoint_refs(all_prefs: dict, ep_id: str) -> int:
     return cleared_users
 
 
+def _default_endpoint_needs_assignment(current_default_id: str, enabled_endpoint_ids) -> bool:
+    """Whether the global default chat endpoint should be (re)assigned.
+
+    True when nothing is configured yet, or the configured default no longer
+    resolves to an enabled endpoint (e.g. the user disabled it). Without the
+    second case, adding a new endpoint after disabling the previous default
+    leaves `default_endpoint_id` pointing at the disabled endpoint, so features
+    that read the raw setting (Memory → Tidy) fail with "No default model
+    configured" even though an enabled endpoint exists. See #3586.
+    """
+    if not current_default_id:
+        return True
+    return current_default_id not in enabled_endpoint_ids
+
+
 # Loopback hosts a user might type for a local model server (LM Studio,
 # llama.cpp, vLLM, …). Inside Docker these point at the *container*, not the
 # host the server actually runs on.
@@ -1727,12 +1742,19 @@ def setup_model_routes(model_discovery):
             )
             db.add(ep)
             db.commit()
-            # Auto-set as default chat endpoint if none configured yet. Seed
-            # the first CHAT model (not raw model_ids[0]) so we don't pin the
-            # global default to an embedding/tts/etc. entry a provider happens
-            # to list first.
+            # Auto-set as default chat endpoint when none is usable yet — either
+            # nothing is configured, or the configured default points at an
+            # endpoint that is now missing/disabled (#3586). Seed the first CHAT
+            # model (not raw model_ids[0]) so we don't pin the global default to
+            # an embedding/tts/etc. entry a provider happens to list first.
             settings = _load_settings()
-            if not settings.get("default_endpoint_id"):
+            enabled_ids = {
+                e.id
+                for e in db.query(ModelEndpoint).filter(
+                    ModelEndpoint.is_enabled == True  # noqa: E712
+                ).all()
+            }
+            if _default_endpoint_needs_assignment(settings.get("default_endpoint_id") or "", enabled_ids):
                 from src.endpoint_resolver import _first_chat_model
                 settings["default_endpoint_id"] = ep.id
                 settings["default_model"] = _first_chat_model(model_ids) or ""