community: set default `output_token_limit` value for `PowerBIToolkit` to fix validation error

[vndee]

Description:

This PR sets a default value of output_token_limit = 4000 for the PowerBIToolkit to fix the unintentionally validation error.

Problem:

When attempting to run a code snippet from Langchain's PowerBI toolkit documentation to interact with a PowerBIDataset, the following error occurs:

pydantic.v1.error_wrappers.ValidationError: 1 validation error for QueryPowerBITool
output_token_limit
  none is not an allowed value (type=type_error.none.not_allowed)

Root Cause:

The issue arises because when creating a QueryPowerBITool, the output_token_limit parameter is unintentionally set to None, which is the current default for PowerBIToolkit. However, QueryPowerBITool expects a default value of 4000 for output_token_limit. This unintended override causes the error.

https://github.com/langchain-ai/langchain/blob/17659ca2cdc418436edf2f8c7f50e800dbbf31ca/libs/community/langchain_community/agent_toolkits/powerbi/toolkit.py#L63 https://github.com/langchain-ai/langchain/blob/17659ca2cdc418436edf2f8c7f50e800dbbf31ca/libs/community/langchain_community/agent_toolkits/powerbi/toolkit.py#L72-L79 https://github.com/langchain-ai/langchain/blob/17659ca2cdc418436edf2f8c7f50e800dbbf31ca/libs/community/langchain_community/tools/powerbi/tool.py#L39

Solution:

To resolve this, the default value of output_token_limit is now explicitly set to 4000 in PowerBIToolkit to prevent the accidental assignment of None.