Add retry decorator with exponential backoff

[aviralgarg05]

Summary

Add retry logic with exponential backoff for flaky API calls in adapters.

Tasks

• Create a @retry decorator in agentunit.core.utils.
• Support configurable max_retries, base_delay, max_delay.
• Log retry attempts with backoff duration.
• Apply to built-in adapters that make external API calls.

Acceptance criteria

• Transient failures retry automatically.
• Permanent failures surface after max retries.
• Decorator is reusable for custom adapters.