- Added usage tracking to AwsEventStreamParser
- Parser now stores usage_credits and context_usage_percentage
- Added get_usage() method to retrieve usage data
- Updated handler to pass usage data to response builder
- Non-streaming: Uses actual usage_credits as total_tokens
- Streaming: Includes usage_credits in final chunk
- Falls back to 0 if Kiro API doesn't provide usage data
- Prevents over-estimation when Kiro returns 0 tokens