docs: fix API key lookup in function calling examples

[git-jxj]

The preset and local Function Calling examples in the English and Chinese custom-dataset guides use env.get(...) without defining env, so copying either example raises NameError before evaluation starts.

Import os and read DASHSCOPE_API_KEY with os.getenv, as the other examples on the same pages do.

Validation: executed all four snippets using the real TaskConfig with evaluation mocked; each now builds the expected API configuration. make lint and the CI lite smoke test (mock model) pass.

[Yunnglin]

Thanks for the fix — verified against the docs. The two Function Calling snippets referenced an undefined env, so copying them raised NameError before the run started. Your change to import os + os.getenv('DASHSCOPE_API_KEY') both resolves it and matches the convention already used by the other examples on the same pages. No env.get remains in either file, and CI is green. LGTM 👍