chore: sort Gemini models

[CXwudi]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas (no code changes)
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods (no code changes)

Description

This little PR simply just sorts the Gemini model list with a sensible order.

Now the list looks more appealing and straightforward compared to before.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• No test is required except checking visually