Remove unnecessary graph_context usage in BigQuery

[Yash (yashupadhyayy1)]

Summary

Fixes broken code example in the BigQuery callback handler integration docs.

Closes #3509

Problem

With langchain-google-community==3.0.5, the Use with LangGraph Agent
section raises two errors:

Error 1 — graph_name is not a valid constructor parameter:
TypeError: BigQueryCallbackHandler.init() got an unexpected keyword argument 'graph_name'

Error 2 — graph_context() method does not exist:
AttributeError: 'BigQueryCallbackHandler' object has no attribute 'graph_context'

[screenshot attached below]

Root Cause

The docs example references graph_name and graph_context() which were
never part of the BigQueryCallbackHandler API in v3.0.5.

Actual constructor signature:
BigQueryCallbackHandler(project_id, dataset_id, table_id=None, config=None)

Fix

• Removed non-existent graph_name argument from constructor
• Removed non-existent graph_context() context manager
• Handler is passed directly via config={"callbacks": [handler]}

[github-actions]

Thanks for opening a docs PR, Yash (@yashupadhyayy1)! When it's ready for review, please add the relevant reviewers:

• @mdrxy (Python integrations)

[Yash (yashupadhyayy1)]

mdrx, please take a review on this error with potential solution.