fix(models): stale cross-provider model no longer shows as unavailable in picker (closes #829)

* fix(models): stale cross-provider model no longer shows as unavailable in picker

Two bugs allowed an openai/gpt-5.4-mini stale session model to appear as
'(unavailable)' under a custom provider group for users who never configured
OpenAI (#829).

Backend (api/routes.py): _resolve_compatible_session_model() had a blanket
early-return for active_provider in {custom, openrouter} that skipped all
normalization regardless of whether any catalog group could route the model's
prefix. A custom_providers-only user with a stale openai/... session model
was never corrected. Fixed: only skip normalization when the model prefix is
actually routable (matches a catalog group provider_id, or an openrouter
group is present that can route any provider/model).

Frontend (static/ui.js): renderSession() injected a bare <option> (not in
any <optgroup>) for models not found in the dropdown. renderModelDropdown()
rendered bare options without emitting a group heading, so they visually
inherited the last rendered provider heading — making the stale model appear
to belong to the custom provider group. Fixed: silently reset to the first
available model and fire a PATCH to persist the correction instead of
injecting a misleading (unavailable) option.

5 new tests in test_provider_mismatch.py cover:
- stale openai model cleared when custom_providers-only + no default_model
- stale openai model cleared when custom_providers-only + default_model set
- openrouter model preserved when openrouter group present
- custom/ namespace always preserved
- ui.js no longer injects model_unavailable option

* fix(ui): declare modelSel locally in syncTopbar reset path; fix test assertion

- Use const modelSel=$('modelSelect') instead of undeclared sel in the
  stale-model reset branch of syncTopbar() (caught in Opus review)
- Fix test assertion: or → and for model_unavailable key absence check

---------

Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>

api/routes.py

@@ -208,7 +208,7 @@ def _resolve_compatible_session_model(model_id: str | None) -> tuple[str, bool]:
         return default_model, bool(default_model)
 
     active_provider = _normalize_provider_id(catalog.get("active_provider"))
-    if not active_provider or active_provider in {"custom", "openrouter"}:
+    if not active_provider:
         return model, False
 
     slash = model.find("/")
@@ -223,6 +223,32 @@ def _resolve_compatible_session_model(model_id: str | None) -> tuple[str, bool]:
         return model, False
 
     model_provider = _normalize_provider_id(model[:slash])
+
+    # For custom/openrouter active providers: only skip normalization when the
+    # model's namespace prefix is actually routable by a group in the catalog.
+    # A user who only has custom_providers configured (active_provider="custom")
+    # with a stale session model like "openai/gpt-5.4-mini" would otherwise
+    # never get cleaned up, causing "(unavailable)" to appear in the picker.
+    if active_provider in {"custom", "openrouter"}:
+        # These namespaces are always routable as-is — preserve them.
+        if model_provider in {"", "custom", "openrouter"}:
+            return model, False
+        # Check if any catalog group can actually route this model's prefix.
+        groups = catalog.get("groups") or []
+        routable_provider_ids = {
+            _normalize_provider_id(g.get("provider_id") or "") for g in groups
+        }
+        # openrouter group can route any provider/model namespace
+        has_openrouter_group = any(
+            (g.get("provider_id") or "") == "openrouter" for g in groups
+        )
+        if model_provider in routable_provider_ids or has_openrouter_group:
+            return model, False
+        # Model prefix is not routable — stale cross-provider reference, clear it.
+        if default_model:
+            return default_model, True
+        return model, False
+
     # Skip normalization for models on custom/openrouter namespaces — these are
     # user-controlled and should never be silently replaced.
     if model_provider and model_provider not in {"", "custom", "openrouter"} and model_provider != active_provider and default_model: