fix: comprehensive test fixes for sequential tool calling

- Fixed wrong method name: changed agent.start() to agent.chat()
- Fixed mock target: changed from litellm.completion to OpenAIClient
- Fixed import path to match project structure
- Rewrote mock implementation to properly simulate OpenAI client behavior
- Fixed test expectations to match actual implementation
- Added try-except around imports for better error handling
- Removed unused imports and fixed code structure
- Added contextlib import for proper error suppression
- Fixed all indentation issues and tool name mismatches

Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

Author: claude[bot]