fix: Improve `Any` decode error (#712)
* Improve Any decoding-error.
Was:
TypeError: Could not convert Any to PredictLongRunningResponse
Now:
TypeError: Could not convert
Any[google.ai.generativelanguage.v1main.PredictLongRunningResponse] to
google.ai.generativelanguage.v1beta.PredictLongRunningResponse
* update test
* update error message
* address review feedback
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Co-authored-by: Anthonios Partheniou <[email protected]>