fix: add a timeout to langchain callback handler

[masci]

Description

The StreamingGeneratorCallbackHandler methods only handles events related to the llm lifecycle. If the thread terminates abnormally, then the while loop in get_response_gen will run indefinitely.

Fixes # n/a

Notes:

• since the timeout has a default value, the change is not backward compatible for callers implicitly relying on a longer execution time - they will get a timeout error if this time is longer than the default 120 seconds. We might increase the default timeout to mitigate this problem, but we can't fix it for good.
• given the dependency on langchain, this change doesn't have unit tests.

[logan-markewich]

The fix looks good to me. Although I wonder if this should be moved to LangChainLLM?