Add warning icon for failed fetch tool requests

[Copilot]

Description

Failed fetch requests (404s, redirects) lack visual indication in the UI. Users must read the expanded content to discover errors.

Changes:

• Tool metadata plumbing: Extended IChatToolInvocation with optional toolMetadata field. Flows through ChatToolInvocation class with serialization support.

• Fetch tool error detection: FetchWebPageTool now sets { hasError: true } metadata when WebContentExtractResult status is error or redirect. Defined IFetchWebPageToolMetadata interface for type safety.

• UI rendering: ChatCollapsibleContentPart accepts optional statusIcon (ThemeIcon). When present, renders inline with warning color. ChatToolInvocationPart checks metadata and passes Codicon.warning to ChatResultListSubPart → ChatCollapsibleListContentPart.

• Test coverage: Added test verifying metadata correctly set for error/redirect/success/mixed scenarios.

Example:

// In fetchPageTool.ts
const hasError = webContents.some(e => e.status === 'error' || e.status === 'redirect');
return {
  content: this._getPromptPartsForResults(results),
  toolResultDetails: actuallyValidUris,
  toolMetadata: { hasError }
};

// In chatToolInvocationPart.ts
const metadata = this.toolInvocation.toolMetadata;
const hasError = metadata?.hasError === true;
// Passes warning icon to UI component when hasError is true

Fixes the issue where failed fetches appeared identical to successful ones in collapsed state.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Fetch tool should better indicate in UI when the request fails</issue_title>
<issue_description>1. Ask the fetch tool to make a request that fails with a 404, such as fetch the contents of http://microsoft.com/no-such-link

Bug
Looking at the UI, it's not very clear that this request failed

I'd expect some kind of warning/error icon</issue_description>

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

• Fixes Fetch tool should better indicate in UI when the request fails #268103

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[vs-code-engineering]

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