Don't veto shutdown in web mode (2025.01 backport of https://github.com/posit-dev/positron/pull/6091)

[jmcphers]

This PR is a backport of #6091 to the 2025.01 release branch; see that PR for notes.

[github-actions]

E2E Tests 🚀  ^?
This PR will run tests tagged with: @critical

[sharon-wang]

I had to grab the changes from b4325b6 to run Positron Server Web locally, as I was getting the following error:

Extension host agent listening on 8080

[bootstrap-import] Initialized node_modules redirector for: /Users/sashimi/dev/positron-dev/remote/node_modules

node:internal/process/promises:391
    triggerUncaughtException(err, true /* fromPromise */);
    ^
Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/sashimi/dev/positron-dev/out/vs/platform/instantiation/common/descriptors' imported from /Users/sashimi/dev/positron-dev/out/vs/server/node/pwbHeartbeat.js
    at finalizeResolution (node:internal/modules/esm/resolve:265:11)
    at moduleResolve (node:internal/modules/esm/resolve:933:10)
    at defaultResolve (node:internal/modules/esm/resolve:1169:11)
    at nextResolve (node:internal/modules/esm/hooks:868:28)
    at resolve (file:///Users/sashimi/dev/positron-dev/out/bootstrap-import.js:51:12)
    at nextResolve (node:internal/modules/esm/hooks:868:28)
    at Hooks.resolve (node:internal/modules/esm/hooks:306:30)
    at handleMessage (node:internal/modules/esm/worker:196:24)
    at checkForMessages (node:internal/modules/esm/worker:138:28)
    at process.<anonymous> (node:internal/modules/esm/worker:157:5) {
  code: 'ERR_MODULE_NOT_FOUND',
  url: 'file:///Users/sashimi/dev/positron-dev/out/vs/platform/instantiation/common/descriptors'
}

ℹ️ Maybe in this PR or separately, we can backport the PWB heartbeat-related changes to this branch?

✅ Issue didn't occur in a dev build of Server Web on Mac when:

• reloading the tab
• closing the tab
• creating a project via the Project Wizard
• opening a folder

📈 Plot test failures are due to the plot baseline image fixes not yet backported to the prerelease/2025.01 branch -- that should be resolved as part of #6008.

[jmcphers]

ℹ️ Maybe in this PR or separately, we can backport the PWB heartbeat-related changes to this branch?

Done in #6072!

[petetronic]

I believe we've addressed the baseline image differences in e2e tests on main due to new runner os.

[petetronic]

We want to take this for the 2025.01.1 patch

[sharon-wang]

LGTM!