fix(message): preserve sticky descendants

[EfeDurmaz16]

Summary

• replace MessageContent's overflow-hidden with overflow-x-clip
• preserve horizontal clipping without creating the vertical overflow behavior that breaks sticky descendants
• add a regression assertion for the rendered class list

Closes #420.

Verification

• pnpm install --frozen-lockfile
• pnpm --filter @repo/elements test -- message.test.tsx (ran the browser test suite; 46 files / 951 tests passed)
• git diff --check

Notes

• pnpm check packages/elements/src/message.tsx packages/elements/__tests__/message.test.tsx reports formatting is correct, then fails in oxlint config with Rule 'no-unresolved' not found in plugin 'import'.\n- pnpm --filter @repo/elements exec tsc --noEmit fails on existing upstream test typing errors unrelated to this change, including the pre-existing MessageContent variant test props and several other test mocks.

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