Add Gemini Flash 2.5 05-20 for the Vertex provider

[hi5hi5]

What problem does this proposed feature solve?

Using of Gemini Flash 2.5 05-20 via the Vertex provider

Describe the proposed solution in detail

Please?

Technical considerations or implementation details (optional)

No response

Describe alternatives considered (if any)

No response

Additional Context & Mockups

No response

Proposal Checklist

• I have searched existing Issues and Discussions to ensure this proposal is not a duplicate.
• This proposal is for a specific, actionable change intended for implementation (not a general idea).
• I understand that this proposal requires review and approval before any development work begins.

Are you interested in implementing this feature if approved?

• Yes, I would like to contribute to implementing this feature.

[hannesrudolph]

@daniel-lxs are you taking this?