feat: add gemini-2.5-flash and set as new default

[cleong14]

Description

• Add gemini-2.5-flash to Gemini model choices
• Set gemini-2.5-flash as new default Gemini model

Related Issue(s)

• Fixes [Bug]: Update Gemini model choices #1683

Screenshots

NA

Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I've updated CodeCompanion.has in the init.lua file for my new feature
• I've added test coverage for this fix/feature
• I've updated the README and/or relevant docs pages
• I've run make all to ensure docs are generated, tests pass and my formatting is applied

[fabiodcorreia]

@cleong14 could you also add gemini-2.5-pro and gemini-2.5-flash-lite-preview-06-17 please?
I was planning to do it but since you already have the PR open :)

[olimorris]

All, before I can test this out, can we confirm what the status is regarding chat buffer editing with tool use, inline editing and cmds? I can't just accept new models unless we're certain they work in CodeCompanion.

[fabiodcorreia]

All, before I can test this out, can we confirm what the status is regarding chat buffer editing with tool use, inline editing and cmds? I can't just accept new models unless we're certain they work in CodeCompanion.

Not sure but I think the pro should work because the preview already worked, but the lite you are probably right its a completly new major version of the model with new capabilities.