test(v2/anthropic): drop placeholder import sentinel from cache_test.go

Removes the blank-assign workaround that was only needed because the
anth import was being kept alive for Task 5's use. Task 5 will bring
the import back when it actually references anth.CacheControlTypeEphemeral.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/anthropic/cache_test.go

@@ -4,7 +4,6 @@ import (
 	"testing"
 
 	"gitea.stevedudenhoeffer.com/steve/go-llm/v2/provider"
-	anth "github.com/liushuangls/go-anthropic/v2"
 )
 
 // TestBuildRequest_MultiSystemUsedWhenSystemPresent verifies that after the
@@ -79,6 +78,4 @@ func TestBuildRequest_NoSystemMessage(t *testing.T) {
 	if anthReq.System != "" {
 		t.Errorf("expected empty System string, got %q", anthReq.System)
 	}
-	// Silences the unused import warning in this file until Task 5.
-	_ = anth.CacheControlTypeEphemeral
 }