Align chat context providers with updated ChatContextItem icon typing

[Copilot]

Recent VS Code proposed API updates changed chat context item icon typing, causing pipeline TypeScript compilation failures in LM context providers. This PR updates the affected provider payloads to match current ChatContextItem shape.

• Type contract alignment

  • Replaced icon with iconPath in chat context items built by:
    • IssueContextProvider
    • WorkspaceContextProvider
    • PullRequestContextProvider

• Scope of change

  • Touches only chat context item construction paths in LM context providers.
  • No behavioral changes to context resolution, labels, commands, or serialized context values.

return {
  iconPath: new vscode.ThemeIcon('git-pull-request'),
  label: `#${pr.number} ${pr.title}`,
  modelDescription: 'The GitHub pull request the user is viewing.',
  tooltip: new vscode.MarkdownString(`#${pr.number} ${pr.title}`),
  pr,
  command: {
    command: 'pr.openDescription',
    title: vscode.l10n.t('Open Pull Request')
  }
};

[azure-pipelines]

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[Copilot]

Pull request overview

This PR updates the extension’s LM chat context providers and bundled proposed VS Code typings to align with the latest vscode.ChatContextItem icon contract (moving from icon to iconPath) to fix TypeScript compilation failures.

Changes:

• Updated LM context item construction to use iconPath instead of icon for PR/issue/workspace chat context items.
• Updated proposed ChatContextItem typing and documentation to reflect iconPath?: IconPath.
• Extended ChatSessionProviderOptionModelMetadata with a promo field (not mentioned in the PR description).

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

File	Description
src/lm/pullRequestContextProvider.ts	Uses iconPath for workspace and PR chat context items.
src/lm/issueContextProvider.ts	Uses iconPath for issue chat context items.
src/@types/vscode.proposed.chatSessionsProvider.d.ts	Adds promo metadata under chat session provider option model metadata.
src/@types/vscode.proposed.chatContextProvider.d.ts	Updates ChatContextItem to iconPath?: IconPath and adjusts related docs/comments.

[alexr00]

@rzhao271 I already did this: #8838