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.go

@@ -82,6 +82,7 @@ type UsageTracker struct {
 	TotalInput    int64
 	TotalOutput   int64
 	TotalRequests int64
+	TotalDetails  map[string]int64
 }
 
 // Add records usage from a single request.
@@ -94,6 +95,14 @@ func (ut *UsageTracker) Add(u *Usage) {
 	ut.TotalInput += int64(u.InputTokens)
 	ut.TotalOutput += int64(u.OutputTokens)
 	ut.TotalRequests++
+	if len(u.Details) > 0 {
+		if ut.TotalDetails == nil {
+			ut.TotalDetails = make(map[string]int64)
+		}
+		for k, v := range u.Details {
+			ut.TotalDetails[k] += int64(v)
+		}
+	}
 }
 
 // Summary returns the accumulated totals.
@@ -103,6 +112,20 @@ func (ut *UsageTracker) Summary() (input, output, requests int64) {
 	return ut.TotalInput, ut.TotalOutput, ut.TotalRequests
 }
 
+// Details returns a copy of the accumulated detail totals.
+func (ut *UsageTracker) Details() map[string]int64 {
+	ut.mu.Lock()
+	defer ut.mu.Unlock()
+	if ut.TotalDetails == nil {
+		return nil
+	}
+	cp := make(map[string]int64, len(ut.TotalDetails))
+	for k, v := range ut.TotalDetails {
+		cp[k] = v
+	}
+	return cp
+}
+
 // WithUsageTracking returns middleware that accumulates token usage across calls.
 func WithUsageTracking(tracker *UsageTracker) Middleware {
 	return func(next CompletionFunc) CompletionFunc {