Fixes #7955

Summary

--reconnection-grace-time (#7678) cannot currently deliver what it documents, for two independent reasons:

1. Closing the tab bypasses the grace time. BrowserLifecycleService runs the workbench shutdown on pagehide/beforeunload, disposing the remote connection gracefully. ManagementConnection treats a graceful dispose as a finished client and calls _cleanResources() immediately — the grace time is never consulted, so the session dies with the tab however the flag is set.

2. A second connection cuts it to 5 minutes. Every new connection calls shortenReconnectionGraceTimeIfNecessary() on all disconnected connections, scheduling ProtocolConstants.ReconnectionShortGraceTime. Opening a second tab is enough to shorten a deliberately long grace time.

This adds patches/session-preservation.diff, which makes the configured grace time authoritative.

Changes

lib/vscode/src/vs/workbench/services/lifecycle/browser/lifecycleService.ts

• pagehide persists UI state (storageService.flush(WillSaveStateReason.SHUTDOWN)) instead of unloading.
• A browser-driven unload no longer tears the workbench down. The veto handler is only passed on the browser path, so that is the discriminator: a beforeunload veto is still honoured, and an explicit in-product shutdown (shutdown(), which passes no veto handler) still unloads normally.

lib/vscode/src/vs/server/node/remoteExtensionHostAgentServer.ts

• The grace time is only shortened when it was left at or below ProtocolConstants.ReconnectionGraceTime. Installations that never set the flag keep Code's stock behaviour; only an operator who deliberately raised it opts into the longer wait.

Also updates the unreleased changelog section.

Test plan

• quilt pop -a && quilt push -a — full series applies cleanly with the new patch
• npm run typecheck-client in lib/vscode — passes
• npm run lint:ts — passes
• npm run test:unit — 331/332; the one failure (should unlink a socket before listening on the socket, expecting EACCES) is environmental, from running as root, and exercises src/node/app.ts, which this PR does not touch
• npm run valid-layers-check fails identically with and without this patch (pre-existing, in network.ts / extensionResourceLoader.ts)
• Behaviour verified in production on 4.128.0: with --reconnection-grace-time 2592000, sessions survive closing the browser and reconnect on return

I was not able to run a full product build on the machine I had available (disk), so a CI build is the remaining check.

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