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>

tests/test_provider_mismatch.py

@@ -498,3 +498,133 @@ def test_empty_model_session_does_not_trigger_save(monkeypatch):
         "_normalize_session_model_in_place must not call session.save() when "
         "the session has no stored model — no correction needed, just a fallback."
     )
+
+
+# ── Issue #829: stale cross-provider model on custom_providers-only setup ─────
+
+def test_stale_openai_model_cleared_for_custom_only_provider(monkeypatch):
+    """A stale openai/... session model must be cleared when active provider is
+    'custom' and no catalog group can route the openai prefix (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    # No routable group for openai/ — should clear to default (empty → model itself
+    # only if no default available, which means changed=False when default_model="")
+    # When default_model is empty, we can't clear — preserve and return False
+    assert changed is False
+    assert effective == "openai/gpt-5.4-mini"
+
+
+def test_stale_openai_model_cleared_for_custom_provider_with_default(monkeypatch):
+    """When active_provider='custom', no openrouter group, and default_model is
+    configured, stale openai/... model should be cleared to default (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "agent37/default",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    assert changed is True
+    assert effective == "agent37/default"
+
+
+def test_openrouter_model_preserved_when_openrouter_group_present(monkeypatch):
+    """When active_provider='openrouter' and openrouter group exists,
+    openai/... model IDs must pass through unchanged — they are routable (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "openrouter",
+            "default_model": "openai/gpt-5.4-mini",
+            "groups": [
+                {"provider": "OpenRouter", "provider_id": "openrouter",
+                 "models": [{"id": "openai/gpt-5.4-mini", "label": "GPT-5.4 Mini"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "openai/gpt-5.4-mini"
+    )
+
+    assert changed is False
+    assert effective == "openai/gpt-5.4-mini"
+
+
+def test_custom_namespace_model_always_preserved_on_custom_provider(monkeypatch):
+    """Model IDs with 'custom/' prefix must always pass through unchanged even
+    when active_provider='custom' (#829)."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "custom",
+            "default_model": "agent37/default",
+            "groups": [
+                {"provider": "Agent37", "provider_id": "custom:agent37",
+                 "models": [{"id": "agent37/default", "label": "default"}]},
+            ],
+        },
+    )
+
+    effective, changed = routes._resolve_compatible_session_model(
+        "custom/my-local-llm"
+    )
+
+    assert changed is False
+    assert effective == "custom/my-local-llm"
+
+
+def test_stale_ui_js_does_not_inject_unavailable_option():
+    """renderSession() must no longer inject a bare (unavailable) option into
+    modelSelect when the session model is not in the provider list (#829).
+    It should silently reset to the first available model instead."""
+    import os
+    src = open(os.path.join(os.path.dirname(__file__), "..", "static", "ui.js"),
+               encoding="utf-8").read()
+
+    # The old pattern must be gone — both keys removed from ui.js
+    assert "model_unavailable" not in src and "model_unavailable_title" not in src, (
+        "renderSession() must not inject '(unavailable)' options — "
+        "stale models should be silently reset to the first available model (#829)"
+    )
+
+    # The new silent-reset pattern must be present
+    assert "first.value" in src and "S.session.model=first.value" in src, (
+        "renderSession() must silently reset S.session.model to the first "
+        "available option when the session model is not in the dropdown (#829)"
+    )