fix: memory leak in notebook editor widget

[SimonSiefke]

Fixes #204756

For testing:

• The number of new ScopedContextKeyWidgets after opening and closing a notebook editor 97 times is reduced from 194 to 0.
• The number of leaked objects after opening and closing a notebook editor 97 times is reduced from ~10000 to ~6000

Test script (scoped context key widget count)

(replace VSCODE_PATH with path to local vscode)

git clone [email protected]:SimonSiefke/vscode-memory-leak-finder.git &&
cd vscode-memory-leak-finder &&
git checkout v5.41.0 &&
npm ci &&
VSCODE_PATH="/home/simon/.cache/repos/vscode/scripts/code.sh"  node packages/cli/bin/test.js --cwd packages/e2e  --check-leaks --measure-after --measure growing-disposable-stores --only ^notebook.open --runs 97 &&
cat .vscode-memory-leak-finder-results/growing-disposable-stores/notebook.open.json

Test script (object count)

(replace VSCODE_PATH with path to local vscode)

git clone [email protected]:SimonSiefke/vscode-memory-leak-finder.git &&
cd vscode-memory-leak-finder &&
git checkout v5.41.0 &&
npm ci &&
VSCODE_PATH="/home/simon/.cache/repos/vscode/scripts/code.sh"  node packages/cli/bin/test.js --cwd packages/e2e  --check-leaks --measure-after --measure object-count --only ^notebook.open --runs 97 &&
cat .vscode-memory-leak-finder-results/object-count/notebook.open.json

[rebornix]

Thank you!