fix(registry): let an alias win its own /v1/models entry

resolve gives an exact alias match precedence over every discovered bare
model id, but listFor did the opposite: it skipped the alias whenever a
discovered model of the same bare id had already been listed. The catalog
then advertised the shadowed provider as the owner, with that provider's
metadata, for an id every request routed to the alias target instead.

A live fleet shows the effect. The alias "deepseek-v4-flash-vision-exp"
targets "gx10/current" (context 1048576), while a cloud provider discovers
a model of the identical bare id. GET /v1/models returned owned_by
"deepseek" and no context_window at all, so clients fell back to their own
128k default while the request ran against a 1M-context local model.

listFor now resolves every usable alias up front and suppresses the bare
entry of any id an alias shadows. The shadowed model stays listed in
"provider/id" form, which is how a client must address it once the alias
owns the bare id, so no model disappears from the catalog.

Claude-Session: https://claude.ai/code/session_0168ukk3uwp8PgiuRAzeLPXp

registry_test.go

@@ -1785,3 +1785,73 @@ func TestModelRegistry_ListFor_Alias_DedupedAgainstDiscoveredCollision(t *testin
 		t.Errorf("owned_by = %q, want %q (the alias's own target provider)", ownedBy, "openai")
 	}
 }
+
+// TestModelRegistry_ListFor_Alias_ShadowedCollisionKeepsAliasEntry proves the
+// listing follows resolve's OWN precedence when a discovered model's bare id
+// collides with a configured alias id: the single bare entry must be the
+// ALIAS's (its target's owner and its resolved metadata), never the
+// discovered model's, because every request for that id lands on the alias
+// target instead (resolve's doc comment). The shadowed provider's own copy
+// stays listed in "provider/id" form so it remains both visible and
+// addressable.
+//
+// Live-fleet regression this fixes (2026-09-10): the alias
+// "deepseek-v4-flash-vision-exp" -> "gx10/current" was listed as the cloud
+// provider's identically-named model, owned_by that cloud provider and
+// carrying NO context_window at all, while every request for the id ran
+// against the local 1M-context model — so clients silently fell back to their
+// own 128k default.
+//
+// Mutation that must make this test fail: restore listFor's old
+// listed[alias]-keyed skip, which emitted the discovered model's entry and
+// dropped the alias's.
+func TestModelRegistry_ListFor_Alias_ShadowedCollisionKeepsAliasEntry(t *testing.T) {
+	t.Parallel()
+	adapters := map[string]providerAdapter{
+		"acme":  newFakeAdapter("acme"),
+		"local": newFakeAdapter("local"),
+	}
+	cfg := &Config{
+		Providers: map[string]*ProviderConfig{
+			"acme":  {Discovery: true},
+			"local": {Models: []string{"current"}},
+		},
+		ModelAliases: map[string]string{"shared-id": "local/current"},
+		ModelMeta: map[string]*ModelMetaConfig{
+			"local/current": {ContextTokens: 1048576, Free: true},
+		},
+	}
+	reg, err := newModelRegistry(adapters, cfg, func(string, ...any) {})
+	if err != nil {
+		t.Fatalf("newModelRegistry: %v", err)
+	}
+	// acme's discovery finds a model literally named "shared-id" — the same
+	// string as the configured alias, discovered only after construction, so
+	// validateModelAliases never saw it.
+	reg.states["acme"].finishRefresh(reg.now(), []string{"shared-id"}, nil)
+
+	got := reg.listFor(allowAllGroup())
+	byID := make(map[string]map[string]any, len(got))
+	bareCount := 0
+	for _, entry := range got {
+		id, _ := entry["id"].(string)
+		byID[id] = entry
+		if id == "shared-id" {
+			bareCount++
+		}
+	}
+	if bareCount != 1 {
+		t.Fatalf("listFor has %d entries for id %q, want exactly 1: %v", bareCount, "shared-id", got)
+	}
+
+	bare := byID["shared-id"]
+	if bare["owned_by"] != "local" {
+		t.Errorf("owned_by = %v, want %q (the alias's own target provider, matching resolve)", bare["owned_by"], "local")
+	}
+	if bare["context_window"] != 1048576 {
+		t.Errorf("context_window = %v, want 1048576 (the alias target's metadata)", bare["context_window"])
+	}
+	if _, ok := byID["acme/shared-id"]; !ok {
+		t.Errorf("listFor = %v, want the shadowed discovered model listed as %q", got, "acme/shared-id")
+	}
+}