Fix worktree context menu items no-op for Copilot CLI sessions created from terminal

[Copilot]

When a CLI session is created from an integrated terminal, the CLI manages worktree creation independently—setWorktreeProperties is never called by the extension, so getWorktreePath(sessionId) returns undefined and all worktree context menu commands silently no-op.

Changes

• IChatSessionWorktreeService + ChatSessionWorktreeService: Add detectAndRegisterWorktreeFromPath(sessionId, path) that checks the in-memory store and metadata store first, then falls back to gitService.getRepository(path). If the path is a git worktree (kind === 'worktree'), it derives ChatSessionWorktreeProperties using worktrees[0] as the parent repo (always first in git worktree list output) and persists them via setWorktreeProperties.

• CopilotCLIChatSessionItemProvider._toChatSessionItem: When getWorktreeProperties returns undefined and session.workingDirectory is set, call detectAndRegisterWorktreeFromPath. This runs during sessions-list rendering—before the user can invoke any context menu—so subsequent getWorktreePath calls succeed.

• Tests: 8 new unit tests covering the detection/caching flow and edge cases (non-worktree paths, missing git repo, already-registered properties, metadata store fallback, empty HEAD).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Open Worktree context menu items no-op for Copilot CLI sessions from terminal</issue_title>
<issue_description>

• + a new Copilot CLI session to be used in an integrated terminal
• See the session in the sessions list
  • My session is Implement Theme Toggle
• ❓ Right-clicking to use the worktree context menu options no-ops
• The worktree context menu options work for a CLI session created in the chat panel
  • So in recording below, it works for add a theme toggle that I created via the chat panel

https://github.com/user-attachments/assets/93900a97-080f-41f5-90ec-d6a16c8ef02f</issue_description>

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

• Fixes Open Worktree context menu items no-op for Copilot CLI sessions from terminal vscode#297829

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