feat/cody: Brings image modality for BYOK users

[PriNova]

This PR brings image modality for BYOK users via the Google LLM provider.

The PR is behind the cody.dev.models experimental feature flag. You need to configure it in the settings.json like this:

"cody.dev.models": [
    {
            "provider": "google",
            "model": "gemini-2.0-flash-exp",
            "inputTokens": 1048576,
            "outputTokens": 8192,
            "apiKey": "your_key_goes_here",
            "options": {
                "temperature": 0.0
            }
        },
 ]

Image_Modality.mp4

Model Selection overhauled:

Test plan

Build Cody based on this PR

Manual Testing Steps

1. Model Selection:

   • Open Cody chat
   • Click on model selector
   • Verify Gemini Flash 2.0 model shows as Vision model

2. Image Upload Flow:

   • Select a Gemini Flash 2.0 model
   • Verify image selection button is visible in toolbar
   • Click image button
   • Select an image file

3. Chat Interaction:

   • With uploaded image, send a message
   • Check response includes image context

4. Edge Cases:

   • Switch between models and verify image selection button visibility
   • Test with various image formats ( jpeg, png, webg )

5. Drag 'n' Drop:

   • Switch to Gemini Flash 2.0
   • Drag an image file from a folder into the Chat Box (need to press SHIFT while dragging Drop files from Explorer to CustomEditor (webview) doesn't work microsoft/vscode#182449)
   • Check that image selection button changes to filename

Notes

• Feature only available for Gemini Flash 2.0 model

Changelog

Added

• Add Gemini Flash 2.0 experimental vision model support via cody dev models flag pull/6354

[abeatrix]

Suggested change

	private images: ImageData[] = []
	private images: ImageData[] = []

I did it this way to get the prototype demo-ready as my hackathon project but I don't think this is the best approach (my bad!).
Instead of passing it to ChatBuidler, could we add a new ContextItem type for media data instead so the images could be preserve in chat history?

[PriNova]

I have also thought about this and find this a great idea. Not only would the user have visual feedback, but there would also be multiple media blobs available in the future.
In its current state, however, this would collide with the other non-visual models, which would mean adding an additional context filter later on (totally feasible).

Additionally, I'm just not sure if a large number of images in the chat history would hurt performance. Depending on the specification of the computer, a slowdown in chat history management is observed: https://linear.app/sourcegraph/issue/CODY-4516/vscode-cody-extension-lags-with-large-chat-history-40-items

[ykdojo]

Tried it again, and it looks great! Copying and pasting is still not working for me, though