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/generate.go

@@ -13,14 +13,16 @@ const structuredOutputToolName = "structured_output"
 // Generate sends a single user prompt to the model and parses the response into T.
 // T must be a struct. The model is forced to return structured output matching T's schema
 // by using a hidden tool call internally.
-func Generate[T any](ctx context.Context, model *Model, prompt string, opts ...RequestOption) (T, error) {
+// Returns the parsed value, token usage, and any error.
+func Generate[T any](ctx context.Context, model *Model, prompt string, opts ...RequestOption) (T, *Usage, error) {
 	return GenerateWith[T](ctx, model, []Message{UserMessage(prompt)}, opts...)
 }
 
 // GenerateWith sends the given messages to the model and parses the response into T.
 // T must be a struct. The model is forced to return structured output matching T's schema
 // by using a hidden tool call internally.
-func GenerateWith[T any](ctx context.Context, model *Model, messages []Message, opts ...RequestOption) (T, error) {
+// Returns the parsed value, token usage, and any error.
+func GenerateWith[T any](ctx context.Context, model *Model, messages []Message, opts ...RequestOption) (T, *Usage, error) {
 	var zero T
 
 	s := schema.FromStruct(zero)
@@ -36,7 +38,7 @@ func GenerateWith[T any](ctx context.Context, model *Model, messages []Message,
 
 	resp, err := model.Complete(ctx, messages, opts...)
 	if err != nil {
-		return zero, err
+		return zero, nil, err
 	}
 
 	// Find the structured_output tool call in the response.
@@ -44,11 +46,11 @@ func GenerateWith[T any](ctx context.Context, model *Model, messages []Message,
 		if tc.Name == structuredOutputToolName {
 			var result T
 			if err := json.Unmarshal([]byte(tc.Arguments), &result); err != nil {
-				return zero, fmt.Errorf("failed to parse structured output: %w", err)
+				return zero, resp.Usage, fmt.Errorf("failed to parse structured output: %w", err)
 			}
-			return result, nil
+			return result, resp.Usage, nil
 		}
 	}
 
-	return zero, ErrNoStructuredOutput
+	return zero, resp.Usage, ErrNoStructuredOutput
 }