Add structured output support with Generate[T] and GenerateWith[T]

Generic functions that use the "hidden tool" technique to force models
to return structured JSON matching a Go struct's schema, replacing the
verbose "tool as structured output" pattern.

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

v2/errors.go

@@ -14,4 +14,7 @@ var (
 
 	// ErrStreamClosed is returned when trying to read from a closed stream.
 	ErrStreamClosed = errors.New("stream closed")
+
+	// ErrNoStructuredOutput is returned when the model did not return a structured output tool call.
+	ErrNoStructuredOutput = errors.New("model did not return structured output")
 )