fix(generative-ai): Use the non stream method instead

[gericdong]

Description

Fixes b/325521336

Streaming method was used incorrectly in a non streaming sample.

Checklist

• [x] Tests pass: npm test (see Testing)
• [x] Lint pass: npm run lint (see Style)
• [x] Please merge this PR for me once it is approved

[iennae]

seems like https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/main/generative-ai/snippets/nonStreamingMultipartContent.js also is using generateContentStream which seems like the fix you're putting forward here. (I got distracted with the changes to the grounding sample that don't seem to be to do with the nonstream method.