test(langchain): add callback propagation test for agent model nodes

[iammannu]

Fixes langchain-ai#36395

Summary

Adds a unit test to verify that callbacks propagate correctly through agent model nodes.

Changes

• Added async unit test for callback propagation in create_react_agent
• Ensures on_llm_start is triggered during agent execution

Why this matters

Prevents regressions where callback chains silently break, affecting observability tools like LangSmith, Langfuse, and OpenTelemetry.

Verification

• Test confirms on_llm_start is triggered during agent execution

[iammannu]

Happy to make any changes if needed. Thanks for reviewing!