[BFCL Chore] Ensure Correct Input Format for Eval Checker

[HuanzhiMao]

In some cases, the model handler’s decode_ast method returns successfully but produces output in an unexpected format, causing issues in downstream evaluations that do not perform argument format validation. This problem is especially common when the model does not output any function calls, resulting in a human-readable string instead of the expected structure.

This PR refines the is_function_calling_format_output function to enforce that outputs must be a list of dictionaries in the following format before calling the checker function:

[
  {func1: {param1: val1, param2: val2, ...}},
  {func2: {param1: val1, param2: val2, ...}},
  ...
]

Note: This PR will not affect the leaderboard score.