Fixes #307446

Problem

The image carousel's bottom caption shows raw markdown instead of readable text, e.g.:

Viewed image [](file:///Users/pierceboggan/Documents/GitHub/coloring-book/homepage.png)

Root cause:

• Tool invocations carry invocationMessage/pastTenseMessage typed string | IMarkdownString. Copilot's view-image tool produces markdown like Viewed image [](file:///…/homepage.png) (the empty-text link renders as a file widget in chat, but is raw syntax anywhere else).
• chatImageExtraction.ts flattened the message to its raw markdown source via .value and stored that as the image caption.
• The carousel editor renders captions as plain text (captionText.textContent = …, per its caption?: string contract), so the markdown syntax was displayed verbatim.

Fix

Preserve the message as string | IMarkdownString in chatImageExtraction.ts (common layer), and convert it to display text at the presentation layer in chatImageCarouselService.ts with a small toCaptionText() helper:

stripIcons(renderAsPlaintext(caption, { useLinkFormatter: true }))

This is the same conversion the chat UI already applies when rendering these tool messages as labels (see chatToolProgressPart.ts), so the carousel caption now matches what the user sees in chat: Viewed image homepage.png (useLinkFormatter resolves empty-text links to the file basename).

Notes:

• The conversion lives in the browser layer because renderAsPlaintext is in vs/base/browser and chatImageExtraction.ts is common-layer code.
• The carousel editor (imageCarouselEditor.ts) is intentionally untouched — its plain-text caption contract is correct; the chat layer was handing it markdown.
• Plain-string messages pass through verbatim; images without a tool message still show no caption.

Tests

• Added a regression test in chatImageCarouselService.test.ts (strips markdown from tool invocation message captions) using the exact message shape from the issue, plus a passthrough assertion for plain-string messages.
• Updated chatImageExtraction.test.ts assertions for the caption type change (captions from IMarkdownString messages now assert on .value).

Before / After

Before:

After: