Change serialization inside GenAIGeminiCreateBatchJobOperator

[Crowiant]

Fix #61116

Found serialization error on Airflow 2. This PR will unblock the release of a new version of google provider.

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[shahar1]

I just figured out that we are lacking unit tests for gen_ai operators module entirely.
I'll be happy if you could at some for the modified methods as part of this PR.

For the rest of the operators - I created #61261 (I'd like to handle it before the next release, to prevent potential regressions in the future).
Thank you!

CC: @VladaZakharova @MaksYermak