fix(langchain_v1): initialize variables in _fetch_last_ai_and_tool_messages

[gowtham1412-p]

Fixes the UnboundLocalError in _fetch_last_ai_and_tool_messages function where variables last_ai_index and last_ai_message were used before being assigned when no AIMessage exists in the messages list.

Changes

• Initialize last_ai_index and last_ai_message to None
• Add validation to raise a clear ValueError if no AIMessage is found
• Prevents UnboundLocalError and provides better error messaging

Issue

Fixes #33696

Dependencies

None

Testing

• Ran make format, make lint, and make test from libs/langchain_v1
• All tests pass