fix(playground): improve Compare response scrolling and copy actions

[ventulus95]

Summary

• constrain the Compare grid and flex height chain so every model response body owns its vertical overflow while preserving the horizontal layout
• keep the model name, status, metrics, and actions outside the scrolling response body
• cover long-response independent scrolling and user-controlled streaming follow behavior in Playwright
• add a per-model copy action beside the remove action, including disabled and success states
• remove the Compare component tests from the Vitest exclusion list so the regression suite actually runs

Root cause

The response body already used overflow-y-auto, but its grid/flex ancestors did not consistently allow shrinking. Without min-h-0 through the height chain and a bounded grid row, the response content expanded the panel instead of producing an internal scroll area.

Before the fix, browser measurements showed clientHeight === scrollHeight === 4308 for long output even though computed overflow was auto.

Validation

• Compare component Vitest suite: 20 tests passed
• focused Playwright tests in system Google Chrome: 2 tests passed
  • long responses scroll independently while headers and metrics stay fixed
  • streaming does not force the user back to the bottom after scrolling up
• npm run lint: passed with only the existing suppression notice
• npm run typecheck:core: passed
• Prettier and git diff --check: passed
• repository pre-commit checks passed for both commits

Fixes #13316

[diegosouzapw]

Thanks for this — the copy-to-clipboard addition on individual Compare columns is a nice,
well-tested UX improvement (reusing the existing useCopyToClipboard hook is exactly right).
Heads up though: the scrolling half of this PR (the min-h-0/overflow changes to
CompareColumn.tsx and CompareTab.tsx) was already fixed and merged separately in #13532, and
the vitest.config.ts exclusion-list hunk is also already gone on the current tip (via #13204).
Could you rebase onto release/v3.8.51 so the diff narrows down to just the copy-button feature
and its test? That should resolve the current merge conflict too. Full credit for the feature is
yours either way — just want to avoid double-applying/reverting the already-merged scroll fix.

[ventulus95]

Done — I rebased the branch onto the latest release/v3.8.51 and removed the already-merged scrolling and Vitest configuration changes. The diff is now limited to the per-column copy action and its unit tests. Thanks for the guidance!

[diegosouzapw]

Thanks @ventulus95 — merging via the release merge-train. Validated in local merge-train (merge-train-20260918-120911-suite.log) on the devbox @ train tip 8c305709478b7052fb981a75852bbf88fe3a959d with the sibling PRs of this batch: typecheck:core, file-size, complexity, cognitive-complexity, changelog-integrity green; changed-area node:test 310/311 (the single red, hard-lease inventory, reproduces on the pure release tip) + vitest green (fast parity mode — full suite ran today on the tip via the base-red and 3b trains). Merged --admin per merge-gates §4/§7.