Add action to attach diff hunk to chat

[Copilot]

Adds ability to attach a specific diff hunk to chat directly from the hunk toolbar. When reviewing Copilot's code changes, users can now click the attach icon to add that exact change as context for follow-up comments.

Changes

• IModifiedFileEntryChangeHunk interface: Added getModifiedLineRange() to expose the modified line range
• DiffHunkWidget: Implements getModifiedLineRange() returning start/end line numbers from the underlying DetailedLineRangeMapping
• AddHunkToChatAction: New action in ChatEditingEditorHunk menu with:
  • Codicon.attach icon (paperclip)
  • Order 20 (renders after diff toggle at order 10)
  • Attaches hunk range to chat widget and focuses input
• Notebook hunks: Return undefined for line range (not applicable to cell-based changes)

Testing

1. Have Copilot make edits to a file
2. Hover over a diff hunk to reveal the toolbar
3. Click the paperclip icon
4. Verify the hunk's code range is attached to chat and input is focused

Original prompt

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This section details on the original issue you should resolve

<issue_title>Ability to add Copilot-targeted comments on diffs</issue_title>
<issue_description>

It would be helpful to the ability to add a comment to Copilot directly on a diff of a code change it applied. When there are many changes, it takes quite some effort to accurately explain what my comment is referring to. Say we had an icon on the toolbar next to the diff icon, it would make it very convenient to add comments targeting that particular piece of change. Copilot would then figure out how to pass it to the underlying LLM so that it has the necessary context to apply the change.</issue_description>

<agent_instructions>This should be an action in the ChatEditingEditorHunk-menu. Use the AttachSelectionToChatAction action, give it an icon (paperclip?) let it render after the diff action</agent_instructions>

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

• Fixes Ability to add Copilot-targeted comments on diffs #287715

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