Fix Corepack shim and disable it by default

[Mike Wallio (scaryrawr)]

Summary

• disable the Corepack authentication shim by default while keeping it available through corepackAlias
• keep Azure Artifacts authentication available for explicit corepack pnpm and corepack yarn commands when enabled
• make corepack enable and corepack disable target the real Corepack binary directory instead of the protected feature shim directory
• preserve caller-provided --install-directory values
• add regression and default-off coverage and bump the artifacts-helper patch version to 3.1.1

Validation

• bash -n src/artifacts-helper/install.sh src/artifacts-helper/scripts/corepack test/artifacts-helper/test_auth_wait.sh test/artifacts-helper/test_shim_integration.sh test/artifacts-helper/test_fallback_execution.sh
• jq empty src/artifacts-helper/devcontainer-feature.json test/artifacts-helper/scenarios.json
• git diff --check
• Linux container checks for enable, disable, explicit install directories, and unchanged command passthrough

This prevents the shim from affecting existing Corepack installations by default and fixes the opt-in EACCES failure where Corepack attempted to unlink /usr/local/share/codespace-shims/pnpm.

[Mike Wallio (scaryrawr)]

Mark Phippard (@markphip) sorry for all the noise 😓. I ran into issues with corepack pnpm install not authing for the package feed, but then didn't realize adding the corepack shim would break how corepack enable pnpm functions.

This fixes the look up for where corepack does its own shims.

This also disables the corepack shim by default now.