Expand Goss tests for Workbench

[!NOTE]
Moved from rstudio/platform-team#412

Context

Workbench images have goss tests but coverage could be expanded, particularly for session images and IDE-specific functionality.

Current goss test templates

• https://github.com/posit-dev/images-workbench/blob/main/workbench/template/test/goss.yaml.jinja2 (228 lines)
• https://github.com/posit-dev/images-workbench/blob/main/workbench-session/template/test/goss.yaml.jinja2
• https://github.com/posit-dev/images-workbench/blob/main/workbench-session-init/template/test/goss.yaml.jinja2
• https://github.com/posit-dev/images-workbench/blob/main/workbench-positron-init/template/test/goss.yaml

Current coverage

Tests check: users, groups, packages, ports, processes, files, and commands.

Potential expansion areas

• IDE session startup and readiness
• VS Code / Positron extension installation verification
• R, Python, and Quarto runtime integration within sessions
• TinyTeX installation verification
• Session-init sidecar container readiness
• Launcher integration (Slurm, Kubernetes plugin checks)
• Multi-user and home directory configuration
• Workbench license activation and status

Related: #458 (centralize common goss patterns)

Start by reading the four current Goss templates listed in the issue and compare the existing checks with the proposed coverage areas. Review related issue #458 before choosing a focused test area. Done means adding and validating a clearly scoped set of Workbench or session-image checks, with the relevant Goss tests passing.