fix(models): show named custom provider label in model dropdown instead of generic 'Custom'

When a custom_providers entry in config.yaml has a 'name' field (e.g. 'Agent37'),
the web UI model picker now uses that name as the group header instead of the
generic 'Custom' label.

Previously all custom_providers entries were bucketed under 'custom' which
rendered as 'Custom' in the dropdown optgroup — losing the named identity the
user set up during onboarding.

Changes:
- Track named custom providers as 'custom:<slug>' keys internally so multiple
  named providers can coexist as separate groups
- When building model groups, emit each named provider under its own display
  name (e.g. 'Agent37') rather than falling through to the generic label
- Unnamed entries (no 'name' field) still fall back to the 'Custom' group
- When all entries are named, the bare 'Custom' bucket is suppressed

Adds 7 tests covering single named provider, multiple named providers,
multiple models in same named provider, unnamed fallback, and mixed cases.

Fixes #557

tests/test_custom_provider_display_name.py

@@ -0,0 +1,135 @@
+"""
+Tests for named custom provider display in the model dropdown (issue #557).
+
+When a custom_providers entry carries a `name` field (e.g. "Agent37"), the
+web UI model picker should show that name as the group header rather than the
+generic "Custom" label.
+"""
+import api.config as config
+
+
+def _models_with_cfg(model_cfg=None, custom_providers=None, active_provider=None):
+    """Temporarily patch config.cfg, call get_available_models(), restore."""
+    old_cfg = dict(config.cfg)
+    config.cfg.clear()
+    if model_cfg:
+        config.cfg["model"] = model_cfg
+    if custom_providers is not None:
+        config.cfg["custom_providers"] = custom_providers
+    try:
+        return config.get_available_models()
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+
+
+# ── Named provider shows its name in the dropdown ─────────────────────────────
+
+class TestNamedCustomProviderGroup:
+
+    def test_named_provider_uses_name_as_group_header(self):
+        """A custom_provider entry with name='Agent37' should produce
+        a group whose 'provider' key is 'Agent37', not 'Custom'."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom", "base_url": "https://agent37.example.com/v1"},
+            custom_providers=[
+                {"name": "Agent37", "model": "default", "base_url": "https://agent37.example.com/v1"}
+            ],
+        )
+        group_names = [g["provider"] for g in result.get("groups", [])]
+        assert "Agent37" in group_names, (
+            f"Expected 'Agent37' in group names, got {group_names}"
+        )
+
+    def test_named_provider_does_not_produce_generic_custom(self):
+        """When all custom_provider entries have names, no group called 'Custom'
+        should appear alongside them."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom", "base_url": "https://agent37.example.com/v1"},
+            custom_providers=[
+                {"name": "Agent37", "model": "default", "base_url": "https://agent37.example.com/v1"}
+            ],
+        )
+        group_names = [g["provider"] for g in result.get("groups", [])]
+        assert "Custom" not in group_names, (
+            f"Expected no generic 'Custom' group when all entries are named, got {group_names}"
+        )
+
+    def test_named_provider_model_appears_in_its_group(self):
+        """The model ID from the named entry should be inside the named group."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom"},
+            custom_providers=[
+                {"name": "Agent37", "model": "my-llm", "base_url": "https://agent37.example.com/v1"}
+            ],
+        )
+        agent37_group = next(
+            (g for g in result.get("groups", []) if g["provider"] == "Agent37"), None
+        )
+        assert agent37_group is not None, "Expected an 'Agent37' group"
+        model_ids = [m["id"] for m in agent37_group.get("models", [])]
+        assert "my-llm" in model_ids, (
+            f"Expected 'my-llm' in Agent37 group models, got {model_ids}"
+        )
+
+    def test_multiple_named_providers_each_get_their_own_group(self):
+        """Two named custom providers should produce two distinct groups."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom"},
+            custom_providers=[
+                {"name": "Agent37", "model": "fast-model"},
+                {"name": "PrivateProxy", "model": "private-llm"},
+            ],
+        )
+        group_names = [g["provider"] for g in result.get("groups", [])]
+        assert "Agent37" in group_names, f"Expected 'Agent37' group, got {group_names}"
+        assert "PrivateProxy" in group_names, f"Expected 'PrivateProxy' group, got {group_names}"
+        assert "Custom" not in group_names, f"No generic 'Custom' group expected, got {group_names}"
+
+    def test_multiple_models_in_same_named_provider(self):
+        """Multiple entries with the same name should be collapsed into one group."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom"},
+            custom_providers=[
+                {"name": "Agent37", "model": "model-a"},
+                {"name": "Agent37", "model": "model-b"},
+            ],
+        )
+        agent37_groups = [g for g in result.get("groups", []) if g["provider"] == "Agent37"]
+        assert len(agent37_groups) == 1, (
+            f"Expected exactly one 'Agent37' group, got {len(agent37_groups)}"
+        )
+        model_ids = [m["id"] for m in agent37_groups[0].get("models", [])]
+        assert "model-a" in model_ids
+        assert "model-b" in model_ids
+
+
+# ── Unnamed entry still falls back to 'Custom' ─────────────────────────────────
+
+class TestUnnamedCustomProviderFallback:
+
+    def test_unnamed_entry_still_produces_custom_group(self):
+        """A custom_provider entry without a name should still show as 'Custom'."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom"},
+            custom_providers=[
+                {"model": "unnamed-model"}
+            ],
+        )
+        group_names = [g["provider"] for g in result.get("groups", [])]
+        assert "Custom" in group_names, (
+            f"Expected generic 'Custom' group for unnamed entry, got {group_names}"
+        )
+
+    def test_mixed_named_and_unnamed_entries(self):
+        """Named and unnamed entries should appear in their respective groups."""
+        result = _models_with_cfg(
+            model_cfg={"provider": "custom"},
+            custom_providers=[
+                {"name": "Agent37", "model": "named-model"},
+                {"model": "unnamed-model"},
+            ],
+        )
+        group_names = [g["provider"] for g in result.get("groups", [])]
+        assert "Agent37" in group_names, f"Expected 'Agent37' group, got {group_names}"
+        assert "Custom" in group_names, f"Expected 'Custom' group for unnamed entry, got {group_names}"