Fix broken openai import in Azure_OpenAI_Graders notebook

[Copilot]

openai v2.x moved EvalStringCheckGrader out of openai.types into openai.types.graders, renaming it to StringCheckGrader, causing a ModuleNotFoundError on notebook startup.

Changes

• Import: openai.types.eval_string_check_grader.EvalStringCheckGrader → openai.types.graders.string_check_grader.StringCheckGrader
• Usage: Renamed all references from EvalStringCheckGrader to StringCheckGrader

# Before
from openai.types.eval_string_check_grader import EvalStringCheckGrader
oai_string_check_grader = EvalStringCheckGrader(...)

# After
from openai.types.graders.string_check_grader import StringCheckGrader
oai_string_check_grader = StringCheckGrader(...)

Constructor signature is unchanged.

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