Add support for Gemini Flash 2.5

We would like to add support for Code Generation to be able to use Gemini Flash 2.5.

This includes adding support in the AIGW and a feature flag in Rails. For details on the specifics, see:

• https://handbook.gitlab.com/handbook/engineering/ai/code-creation/code-suggestions-guide/implementation_guidelines/

Next Steps

• After reviewing the handbook, update this issue description with more specifics.
• Check the AIGW to look for any existing Gemini integrations