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/response.go

@@ -1,5 +1,7 @@
 package llm
 
+import "gitea.stevedudenhoeffer.com/steve/go-llm/v2/provider"
+
 // Response represents the result of a completion request.
 type Response struct {
 	// Text is the assistant's text content. Empty if only tool calls.
@@ -31,4 +33,45 @@ type Usage struct {
 	InputTokens  int
 	OutputTokens int
 	TotalTokens  int
+	Details      map[string]int // provider-specific breakdown (e.g., cached, reasoning tokens)
 }
+
+// addUsage merges usage u into the receiver, accumulating token counts and details.
+// If the receiver is nil, it returns a copy of u. If u is nil, it returns the receiver unchanged.
+func addUsage(total *Usage, u *Usage) *Usage {
+	if u == nil {
+		return total
+	}
+	if total == nil {
+		cp := *u
+		if u.Details != nil {
+			cp.Details = make(map[string]int, len(u.Details))
+			for k, v := range u.Details {
+				cp.Details[k] = v
+			}
+		}
+		return &cp
+	}
+	total.InputTokens += u.InputTokens
+	total.OutputTokens += u.OutputTokens
+	total.TotalTokens += u.TotalTokens
+	if u.Details != nil {
+		if total.Details == nil {
+			total.Details = make(map[string]int, len(u.Details))
+		}
+		for k, v := range u.Details {
+			total.Details[k] += v
+		}
+	}
+	return total
+}
+
+// Re-export detail key constants from provider package for convenience.
+const (
+	UsageDetailReasoningTokens     = provider.UsageDetailReasoningTokens
+	UsageDetailCachedInputTokens   = provider.UsageDetailCachedInputTokens
+	UsageDetailCacheCreationTokens = provider.UsageDetailCacheCreationTokens
+	UsageDetailAudioInputTokens    = provider.UsageDetailAudioInputTokens
+	UsageDetailAudioOutputTokens   = provider.UsageDetailAudioOutputTokens
+	UsageDetailThoughtsTokens      = provider.UsageDetailThoughtsTokens
+)