fix(session-ui): align code copy tooltip with v2 style

[opencode-agent]

Summary

• Restyle markdown code-block copy tooltip to match TooltipV2 tokens in the new layout
• Preserve the legacy tooltip appearance under body:not([data-new-layout])

Validation

• bun typecheck from packages/session-ui
• bun typecheck from packages/app
• git diff --check
• Production session timeline benchmark before/after (timeline/session-timeline-benchmark.spec.ts)
  • before: passed, completionObservedMs 64107.8, rowReplaced false, markdownReplaced false
  • after: passed, completionObservedMs 64372.9, rowReplaced false, markdownReplaced false
• Real-app before/after screenshot + pixel diff captured with mocked backend fixture; changed region limited to tooltip (1297 / 189440 pixels, 0.685%)

Notes

• Chromium needed cached runtime libs via LD_LIBRARY_PATH=/home/gangprompt/services/opencord/agent-root/output/browser-libs/usr/lib for local Playwright runs.