test(v2): end-to-end cache-hint propagation through Chat.Send

Verifies that WithPromptCaching() on a Chat results in CacheHints being
set on the provider.Request that reaches the provider layer, and that
omitting the option leaves CacheHints nil (no behavior change for
existing callers).

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

v2/chat_test.go

@@ -509,3 +509,59 @@ func TestChat_Send_UsageWithDetails(t *testing.T) {
 		t.Errorf("expected reasoning_tokens=2, got %d", usage.Details["reasoning_tokens"])
 	}
 }
+
+// TestChatSend_WithPromptCaching_PopulatesCacheHints is an end-to-end check
+// that WithPromptCaching() on a Chat causes CacheHints to be populated on the
+// provider.Request that reaches the provider layer.
+func TestChatSend_WithPromptCaching_PopulatesCacheHints(t *testing.T) {
+	mp := newMockProvider(provider.Response{Text: "hello back"})
+	model := newMockModel(mp)
+
+	chat := NewChat(model, WithPromptCaching())
+	chat.SetSystem("you are helpful")
+
+	_, _, err := chat.Send(context.Background(), "hi")
+	if err != nil {
+		t.Fatalf("Send failed: %v", err)
+	}
+
+	if len(mp.Requests) != 1 {
+		t.Fatalf("expected 1 provider request, got %d", len(mp.Requests))
+	}
+	req := mp.Requests[0]
+
+	if req.CacheHints == nil {
+		t.Fatal("expected CacheHints to be set on provider.Request")
+	}
+	if !req.CacheHints.CacheSystem {
+		t.Error("expected CacheSystem=true")
+	}
+	// Last non-system message index should be the user "hi" at index 1
+	// (SetSystem prepended a system message at index 0).
+	if req.CacheHints.LastCacheableMessageIndex != 1 {
+		t.Errorf("expected LastCacheableMessageIndex=1, got %d", req.CacheHints.LastCacheableMessageIndex)
+	}
+}
+
+// TestChatSend_WithoutPromptCaching_NoCacheHints verifies that omitting
+// WithPromptCaching() leaves CacheHints nil on the provider.Request, so
+// existing callers see no behavior change.
+func TestChatSend_WithoutPromptCaching_NoCacheHints(t *testing.T) {
+	mp := newMockProvider(provider.Response{Text: "hello back"})
+	model := newMockModel(mp)
+
+	chat := NewChat(model)
+	chat.SetSystem("you are helpful")
+
+	_, _, err := chat.Send(context.Background(), "hi")
+	if err != nil {
+		t.Fatalf("Send failed: %v", err)
+	}
+
+	if len(mp.Requests) != 1 {
+		t.Fatalf("expected 1 provider request, got %d", len(mp.Requests))
+	}
+	if mp.Requests[0].CacheHints != nil {
+		t.Errorf("expected nil CacheHints when caching not requested, got %+v", mp.Requests[0].CacheHints)
+	}
+}