Filter unsupported image formats for Gemini models

[Copilot]

Gemini models reject GIF images (supported formats: PNG, JPEG, WEBP, HEIC, HEIF). Users hitting this when attaching GIFs or switching models mid-conversation.

Changes

Model capabilities (chatModelCapabilities.ts):

• Added GEMINI_SUPPORTED_IMAGE_MIME_TYPES constant
• Added isGeminiFamily() and modelSupportsImageMimeType() helpers

Image attachments (image.tsx):

• Check format support before rendering, show clear error for unsupported types
• Added getFormatFromMimeType() helper with fallback for malformed MIME types

Message conversion (geminiMessageConverter.ts):

• Filter unsupported images from user messages and tool results using constant

Example:

// New attachments with Gemini
if (imageMimeType && !modelSupportsImageMimeType(endpoint, imageMimeType)) {
  // Shows: "Gemini does not support GIF images. Supported formats: PNG, JPEG, WEBP, HEIC, HEIF."
  return <OmittedReference />;
}

// Conversation history filtering
const isSupported = !mimeType.startsWith('image/') || 
  GEMINI_SUPPORTED_IMAGE_MIME_TYPES.includes(mimeType);

Added 16 unit tests covering model detection and image filtering.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Gemini handling for image/gif needs improvement</issue_title>
<issue_description>Gemini models do not support GIF images, see https://ai.google.dev/gemini-api/docs/image-understanding#supported-formats
We need to make sure that either we don't allow attaching it when Gemini is selected or to convert the image instance on upload to png. When transferring from another model, we also need to drop those image/gif references retroactively.</issue_description>

Comments on the Issue (you are @copilot in this section)

@isidorn @pwang347 any way to try to handle this as part of January? CAPI just raised this as happening often - I can get the data to see how often it really happens, just let me know @vijayupadya @isidorn is the issue CAPI raised about the general image type required issue (err below) OR specifically GIF issue?

Request Failed: 400 {"error":{"message":"image media type is required","code":"invalid_request_body"}}

This above issue is already fixed last week. </comment_new>

• Fixes Gemini handling for image/gif needs improvement vscode#286628

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.