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 <partheniou@google.com>
2 files changed