Merged duplicate `GeminiAdapter` class definitions

[KangmoonSeo]

Why are these changes needed?

This PR addresses the issue of duplicate GeminiAdapter class definitions found in the codebase. The changes aim to merge the two identical class definitions, keeping the more specific implementation of the get_default_conv_template method. This will resolve potential confusion and issues with class resolution.

Changes made

• Removed the duplicate GeminiAdapter class definition
• Updated the remaining GeminiAdapter class to include the more specific get_default_conv_template method

Related issue number (if applicable)

Closes #3462

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).