feat: comprehensive token usage tracking for V2

Add provider-specific usage details, fix streaming usage, and return
usage from all high-level APIs (Chat.Send, Generate[T], Agent.Run).

Breaking changes:
- Chat.Send/SendMessage/SendWithImages now return (string, *Usage, error)
- Generate[T]/GenerateWith[T] now return (T, *Usage, error)
- Agent.Run/RunMessages now return (string, *Usage, error)

New features:
- Usage.Details map for provider-specific token breakdowns
  (reasoning, cached, audio, thoughts tokens)
- OpenAI streaming now captures usage via StreamOptions.IncludeUsage
- Google streaming now captures UsageMetadata from final chunk
- UsageTracker.Details() for accumulated detail totals
- ModelPricing and PricingRegistry for cost computation

Closes #2

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

v2/middleware_test.go

@@ -280,3 +280,80 @@ func TestWithLogging_Error(t *testing.T) {
 		t.Errorf("expected provider error, got %v", err)
 	}
 }
+
+func TestUsageTracker_Details(t *testing.T) {
+	tracker := &UsageTracker{}
+
+	tracker.Add(&Usage{
+		InputTokens:  100,
+		OutputTokens: 50,
+		TotalTokens:  150,
+		Details: map[string]int{
+			"cached_input_tokens": 20,
+			"reasoning_tokens":    10,
+		},
+	})
+
+	tracker.Add(&Usage{
+		InputTokens:  80,
+		OutputTokens: 40,
+		TotalTokens:  120,
+		Details: map[string]int{
+			"cached_input_tokens": 15,
+		},
+	})
+
+	details := tracker.Details()
+	if details == nil {
+		t.Fatal("expected details, got nil")
+	}
+	if details["cached_input_tokens"] != 35 {
+		t.Errorf("expected cached_input_tokens=35, got %d", details["cached_input_tokens"])
+	}
+	if details["reasoning_tokens"] != 10 {
+		t.Errorf("expected reasoning_tokens=10, got %d", details["reasoning_tokens"])
+	}
+
+	// Verify returned map is a copy
+	details["cached_input_tokens"] = 999
+	fresh := tracker.Details()
+	if fresh["cached_input_tokens"] != 35 {
+		t.Error("Details() did not return a copy")
+	}
+}
+
+func TestUsageTracker_Details_Nil(t *testing.T) {
+	tracker := &UsageTracker{}
+	tracker.Add(&Usage{InputTokens: 10, OutputTokens: 5, TotalTokens: 15})
+
+	details := tracker.Details()
+	if details != nil {
+		t.Errorf("expected nil details for usage without details, got %v", details)
+	}
+}
+
+func TestWithUsageTracking_WithDetails(t *testing.T) {
+	mp := newMockProvider(provider.Response{
+		Text: "ok",
+		Usage: &provider.Usage{
+			InputTokens:  100,
+			OutputTokens: 50,
+			TotalTokens:  150,
+			Details: map[string]int{
+				"cached_input_tokens": 30,
+			},
+		},
+	})
+	tracker := &UsageTracker{}
+	model := newMockModel(mp).WithMiddleware(WithUsageTracking(tracker))
+
+	_, err := model.Complete(context.Background(), []Message{UserMessage("test")})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	details := tracker.Details()
+	if details["cached_input_tokens"] != 30 {
+		t.Errorf("expected cached_input_tokens=30, got %d", details["cached_input_tokens"])
+	}
+}