Fix popup-blocked dialog on vscode.dev/agents first load (#312417)

* Fix popup-blocked dialog on vscode.dev/agents first load

Register SessionsWorkspaceContextService and the sessions ConfigurationService in the browser agents workbench, mirroring what the desktop entry does in electron-browser/sessions.main.ts.

The fix:
- Make BrowserMain.initServices() protected so SessionsBrowserMain can override it
- Override initServices() to replace IWorkspaceContextService, IWorkspaceEditingService, and IWorkbenchConfigurationService with the sessions implementations that manage workspace folders in-memory

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix sessions web workspace service initialization

Extract createWorkspaceConfigAndStorageServices() from BrowserMain as a
protected method so SessionsBrowserMain can override just the workspace,
configuration, storage, and trust service creation.

This mirrors the desktop SessionsMain pattern: the agents window builds its
workspace/configuration services from the start, using SessionsWorkspaceContextService
and sessions ConfigurationService. No standard WorkspaceService is ever created.

All dependent services (request, encryption, secrets, user data sync/init) remain
in the base class and are shared by both the standard and sessions web entries.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>