feat(genai): support optional seed parameter

[Odessit007]

Description

Support an optional seed parameter that Google Gen AI SDK exposes for more reproducible generations but langchain-google-genai currently doesn't support.

Relevant issues

Closes #1355

Type

🆕 New Feature

[mdrxy]

Could you please add unit tests?

[mdrxy]

bump @Odessit007

[ivustianiu-illumina]

Hi @mdrxy. I've been sick since last week. I should have time in the next few days to have a look.

Initially I didn't add tests because I noticed that there are no tests for seed parameter in vertexai package and because they'll be flaky (it's a best effort on the Google's side to ensure reproducibility but the nature of hardware and distributed compute may still cause nondeterminism).

Now that you requested, I'm going to add a simple integration test marked as flaky with 3-5 retries similar to existing tests. Also, maybe a unit test to check that the parameter is properly set.

[mdrxy]

gotcha thanks! hope you're doing better

deferring this PR until after the SDK consolidation takes place