[VSCode] Add gitlab.virtualWorkspaceProjects setting for pure-virtual-workspace users

Summary

gitlab-org/editor-extensions/gitlab-lsp!3295 (merged) added a virtualWorkspaceProjects ClientConfig field on the LSP that maps virtual workspace folder URIs (e.g. adt://server/sap/bc/adt/packages/zmy_package) to GitLab project paths.

Companion VS Code support is needed for users with purely virtual workspaces (e.g. SAP ABAP Remote FS without a local Git repo alongside). Without it, AgentPlatformProjectService.getRepositories() returns empty, the webview's isDuoEnabledForAnyProject returns false, and Duo agent chat is disabled.

Surface a gitlab.virtualWorkspaceProjects user/workspace setting in the VS Code extension that is forwarded to the LSP via the existing didChangeConfiguration flow, allowing customers to configure the URI to project path mapping themselves.

Driver: customers using https://github.com/marcellourbani/vscode_abap_remote_fs in pure-virtual-workspace mode.