fix bedrock connection error due to missing description

[JoseCelis]

Docstrings included in timecopilot/agent.py in tsfeatures_tool, cross_validation_tool and forecast_tool.
These docstrings are required by pydantic.
These changes solve the issue:

raise ModelHTTPError(status_code=status_code, model_name=self.model_name, body=e.response) from e
pydantic_ai.exceptions.ModelHTTPError: status_code: 400, model_name: '', body: {'Error': {'Message': 'The model returned the following errors: {"error":{"code":"validation_error","message":"ErrorEvent { error: APIError { type: \\"BadRequestError\\", code: Some(400), message: \\"1 validation error for ToolDescription\\\\ndescription\\\\n  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]\\\\n    For further information visit [https://errors.pydantic.dev/2.12/v/string_type\\](https://errors.pydantic.dev/2.12/v/string_type%5C%5C)", param: None } }","param":null,"type":"invalid_request_error"}}', 'Code': 'ValidationException'}, 'ResponseMetadata': {'RequestId': '', 'HTTPStatusCode': 400, 'HTTPHeaders': {'date': 'Sat, 21 Feb 2026 15:21:32 GMT', 'content-type': 'application/json', 'content-length': '512', 'connection': 'keep-alive', 'x-amzn-requestid': '', 'x-amzn-errortype': 'ValidationException:http://internal.amazon.com/coral/com.amazon.bedrock/'}, 'RetryAttempts': 0}, 'message': 'The model returned the following errors: {"error":{"code":"validation_error","message":"ErrorEvent { error: APIError { type: \\"BadRequestError\\", code: Some(400), message: \\"1 validation error for ToolDescription\\\\ndescription\\\\n  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]\\\\n    For further information visit [https://errors.pydantic.dev/2.12/v/string_type\\](https://errors.pydantic.dev/2.12/v/string_type%5C%5C)", param: None } }","param":null,"type":"invalid_request_error"}}'}

[AzulGarza]

hey @JoseCelis! thanks so much for the contribution. lgtm:)