[Gemini function calling] Fix type annotation for function output in generative-ai/function-calling example

[koverholt]

Description

Fixes #11253.

This is a followup PR to #11254 to fix the type annotation in the function output to be a tuple instead of a string.

Also fixes an issue by changing the function role to user role for consistency w/ the Gemini API.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[koverholt]

Apologies, I mixed up my branches and ended up committing the type annotation fix in 72a2f1c in #11257. 😅

But, I also fixed an issue in this PR related to the function vs. user role to be consistent with the Gemini API.

Ping @m-strzelczyk if you're able to review this (small-ish) PR as well.

FYI, tests are passing for the function calling code samples, but failing for others not related to this PR.