[FEATURE] Per-message copy button in VSCode extension (parity with desktop/macOS app)

The Claude desktop/macOS app shows a copy button on each message (on hover / bottom-left) to copy that message's content to the clipboard. The VSCode extension does not have per-message copy controls at parity.

Request: Add a copy button to each message (both user and assistant) in the chat thread in the VSCode extension, matching the desktop app.

Details:

• Copy the message's raw markdown (preserving code fences, links, and formatting) rather than rendered/plain text, so pastes into docs/PRs/Slack survive intact.
• Show it per-message (hover or persistent), on both user and assistant messages.

Why: Grabbing a full response, a command, or a code block currently requires manual drag-select, which often mangles multi-line content and gutter spacing. The desktop app already solves this; the VSCode extension should match.

Related: #5512, #65129.