Add support for Gemini PHP client 2.x version that supports Structured Output

I followed https://jaybeaton.com/blog/how-use-version-module-drupal-issue-fork-comp... to install the issue fork https://git.drupalcode.org/issue/gemini_provider-3519469 code as a package to the composer.json so it can be installed locally for testing.

Adding tests/ai-test-drush-scripts/GeminiFunctionCallTester.php to test function calls.

QA steps:

1. Checkout the branch from this issue's MR.
2. Run ddev like normal
3. Run ddev poser to install different version of the gemini_provider module and the dependencies google-gemini-php/client and drupal/ai_agents
4. Configure Gemini AI following contributing steps (if haven't done before)
5. Run ddev drush php-script tests/ai-test-drush-scripts/GeminiFunctionCallTester.php
6. Confirm it outpus 140. Change the prompt in the test file and run it again to confirm the calculator function works.

Thanks for doing this, @dmundra! What is your thought re: getting Structured Output added? Does this need to be merged before that can be done?

I think either add it to ✨ Support tools calling Active or copy the changes from that ticket to ✨ Upgrade to 2.0 version of google-gemini-php Active and then add it there.

I'm thinking we might not want to merge this atm -- we will have the update the commit ref in `composer.json` whenever there is a new commit in the MR. I'm thinking we should wait to act until the change is merged in or at least more stable. Thoughts, @dmundra?

I agree @majorrobot

This MR is available to use for testing. Setting it postponed for now.

Sounds good -- thx @dmundra.