feat(adapters): copy stdlib llm package (Client + Router) per ADR-004

Copy hyperguild/ingestion/internal/llm into internal/adapters/llm and own it.
Tapir owes that repo nothing at the dependency level — no module dep added.
Router gives the local-Primary -> BYO-Fallback path needed for ai_routing.feature.

Copied tests rewritten from testify to stdlib testing to keep go.mod
dependency-free (repo has zero deps; acceptance tests are stdlib too).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/llm/router_test.go

@@ -0,0 +1,78 @@
+package llm
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestRouter_PrimarySucceeds(t *testing.T) {
+	primary := mockServer(t, "from-primary")
+	defer primary.Close()
+	fallback := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Error("fallback must not be called when primary succeeds")
+	}))
+	defer fallback.Close()
+
+	r := &Router{
+		Primary:  New(primary.URL, "", "m", time.Second),
+		Fallback: New(fallback.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	if err != nil {
+		t.Fatalf("Complete: %v", err)
+	}
+	if out != "from-primary" {
+		t.Errorf("got %q, want %q", out, "from-primary")
+	}
+}
+
+func TestRouter_FallsBackOnPrimaryError(t *testing.T) {
+	primary := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "unavailable", http.StatusServiceUnavailable)
+	}))
+	defer primary.Close()
+	fallback := mockServer(t, "from-fallback")
+	defer fallback.Close()
+
+	r := &Router{
+		Primary:  New(primary.URL, "", "m", time.Second),
+		Fallback: New(fallback.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	if err != nil {
+		t.Fatalf("Complete: %v", err)
+	}
+	if out != "from-fallback" {
+		t.Errorf("got %q, want %q", out, "from-fallback")
+	}
+}
+
+func TestRouter_BothFail(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fail.URL, "", "m", time.Second),
+	}
+	if _, err := r.Complete(context.Background(), "sys", "user"); err == nil {
+		t.Fatal("want error when both fail, got nil")
+	}
+}
+
+func TestRouter_NilFallback(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{Primary: New(fail.URL, "", "m", time.Second)}
+	if _, err := r.Complete(context.Background(), "sys", "user"); err == nil {
+		t.Fatal("want error with nil fallback, got nil")
+	}
+}