workbench.experimental.modernUI on by default w/o a clear instruction how to turn it off

Type: Bug

Ugly rounded corners enabled without a clear communication how to turn them off. I had to search through Internet forums for the specific UI option key, which was not obvious at all.

VS Code version: Code 1.134.0 (Universal) (110a328ea54b42367b803ec53ee0bf52ef26b419, 2026-08-18T18:24:44Z)
OS version: Darwin arm64 22.6.0
Modes:
Remote OS version: Linux x64 6.8.0-124-generic
Remote OS version: Linux x64 7.0.0-29-generic

A/B Experiments

```
vsliv368:30146709
pythonvspyt551:31249599
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
envsactivate1:31551504
gh487529:31544284
cloudbuttont:31379625
3efgi100_wstrepl:31403338
ec5jj548:31422691
cp_cls_t_966_ss:31526232
c0683394:31568964
8hhj4413:31478653
cp_cls_c_1081:31454833
conptydll_true:31498968
e9c30283:31461165
test_treatment2:31471001
46204921:31447328
ei9d7968:31496641
chat:31457767
ghj88844:31499326
ddid_t:31478206
hmra_i5g22:31518061
bjc72774_agent_sandbox:31557830
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
j0d79568:31499440
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
jh5f2457_c:31540921
jbcp_cls_pctr_t:31531130
61138546:31518536
cp_intellij_t_nes:31548657
jf4hg949:31526829
ahp-both-windows:31556933
sessions_5:31532894
ihg5j128:31534457
f412h606:31551416
9gg7f176:31542112
sandbox-ui-on-2:31543161
enable_editor_pane_layout:31569726
allow-none:31555437
6i51c363:31551401
36h42362:31564511
c7c27ce7:31554789
1h923230:31564177
0dg73464:31554319
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31570291
a1hcc538:31559322
0c1h4866:31566224
vrbsty_tr:31561058
session-mark-done:31558131
5b8j3302:31564601

```