refactor(model-routing): centralize explicit foreground fallback policy (#6020)

* refactor(model-routing): centralize explicit foreground fallback policy

Make foreground fallback an explicit per-user, availability-only policy shared by streaming Chat, non-stream Chat, and Agent runs.

Preserve strict defaults, owner/model and credential boundaries, pinned Agent routes, and truthful per-round provenance/accounting. Carry provider-reported model identifiers through native streaming adapters, non-stream responses, and caches, and keep legacy default_model_fallbacks as tombstoned raw storage that generic settings APIs and agent tools cannot expose or mutate.

* fix(agent-loop): restore rebase-dropped qwen routing, workspace prompt, and temperature clamp

* fix(model-routing): thread selected endpoint identity, fix cost classification and fallback eligibility

* fix(chat): restore stream helpers and harden run stop lifecycle

* fix(model-routing): let numeric provider codes win over symbolic rate-limit statuses

* fix(agent-loop): apply qwen temperature and notes-tool clamps per fallback candidate

* fix(chat): honor queued stop across resend and reload canonical terminal on EOF

* fix(chat): track stop queue and cleanup ownership by per-send generation

* fix(agent-loop): preserve requested temperature for non-qwen fallback candidates

* fix(chat): reserve send ownership before any await and scope stop to the current send

* fix(chat): clear the previous run identity at send reservation

---------

Co-authored-by: RaresKeY <158580472+RaresKeY@users.noreply.github.com>
Co-authored-by: StressTestor <212606152+StressTestor@users.noreply.github.com>

routes/model_routes.py

@@ -46,6 +46,7 @@ _ENDPOINT_SETTING_FIELDS = {
 }
 
 _ENDPOINT_FALLBACK_FIELDS = {
+    "foreground_model_fallbacks": "Foreground Model Fallbacks",
     "utility_model_fallbacks": "Utility Model Fallbacks",
     "vision_model_fallbacks":  "Vision Model Fallbacks",
 }
@@ -180,7 +181,12 @@ def _clear_user_pref_endpoint_refs(all_prefs: dict, ep_id: str) -> int:
     if not isinstance(all_prefs, dict):
         return 0
     users = all_prefs.get("_users")
-    pref_sets = users.values() if isinstance(users, dict) else [all_prefs]
+    # A mixed store can contain auth-disabled foreground policy at the root
+    # alongside named-owner preferences. Both are active namespaces; legacy
+    # `default_model_fallbacks` remains untouched by the field allowlist.
+    pref_sets = [all_prefs]
+    if isinstance(users, dict):
+        pref_sets.extend(users.values())
     cleared_users = 0
     for prefs in pref_sets:
         if isinstance(prefs, dict) and _clear_endpoint_settings_for_endpoint(prefs, ep_id):