fix(web): name message copy actions accurately

[saphid]

What Changed

Message copy controls announced “Copy link” to screen readers while copying user or assistant message text. Their accessible name and tooltip now both say “Copy message”, including messages carrying structured context. The existing collapsed-message test expectation follows the corrected name.

Why

This implements the message-copy mismatch covered by the proposed action-names invariant. Other inspected code, plan and native message controls already identify their targets. This small fix does not claim whole-app compliance.

This implementation is stacked on docs/consistency-action-names in the fork; the documentation PR independently targets upstream main.

Validation

• Existing MessagesTimeline.test.tsx: 67 tests passed (CI=true vp test run --project unit src/components/chat/MessagesTimeline.test.tsx, from apps/web).
• Scoped lint for both changed code files exited 0; its pre-existing empty test-class warning remains.
• Independent direct SWE-2 Max review exited 0 with no actionable findings on the frozen two-file change and contract. Launcher: devin -p --model swe-2-max with a file-backed review prompt; read-only review included the repository coding standard.

UI Changes

Runtime proof (2026-09-20, isolated proof app; base d6f2913 vs this branch): the message action buttons expose aria-label="Copy link" on base and aria-label="Copy message" on this branch (DOM accessible-name observations; baseline interaction capture below).

Before: https://github.com/user-attachments/assets/71591567-9fb2-4df5-a232-25eb28f6c00b

After-state evidence is the DOM observation plus a captured context screenshot in the session artifacts; the visible change is the accessible name, not layout.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for UI changes

Implemented by GPT-6 Astra (low reasoning) in Codex; independently reviewed by SWE-2 Max in Devin CLI.

[saphid]

Superseded by pingdotgg#12865 — the implementation PR now lives upstream against main (it contains the docs commit until the paired docs PR merges). Same head branch, same commits; this fork draft is closed to avoid a duplicate review surface.