[tool] fix: geo3k create return

[nanjiangwill]

What does this PR do?

change tool.create return from instance_id, None to instance_id

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[gemini-code-assist]

Code Review

This pull request correctly fixes a return type mismatch in the Geo3kTool.create method. The original implementation returned a tuple, which was inconsistent with the method's type hint (-> str) and the base class BaseTool's contract. The change ensures the method now correctly returns a string instance_id, resolving the bug and improving the code's correctness and robustness. The change is accurate and well-contained. I have no further feedback.