fix(artifacts-helper): restore shim installation into shell rc files

[Copilot]

Two bugs introduced in late 2025 caused install.sh to silently skip writing shell function shims into .bashrc/.zshrc, leaving tools like npm as plain executables rather than intercepted functions — causing 401 auth failures against Azure Artifacts feeds.

Root causes

• Missing variable initialization: COMMA_SEP_TARGET_FILES was never set to "DEFAULT", so the guard if [ "${COMMA_SEP_TARGET_FILES}" = "DEFAULT" ] was always false and target rc files were never populated.
• Broken newlines in shim block: \n inside a bash double-quoted += doesn't produce a real newline, so all function definitions concatenated into one line and were unparseable as shell functions.

Changes

• src/artifacts-helper/install.sh
  • Restore COMMA_SEP_TARGET_FILES="${TARGETFILES:-"DEFAULT"}" before the default-resolution block
  • Fix newline appending: "...\n" → "..."$'\n'

# Before (broken): \n is literal, not a newline
ALIASES_BLOCK+="$ALIAS() { \"${SHIM_DIRECTORY}/$ALIAS\" \"\$@\"; }\n"

# After: $'\n' produces an actual newline
ALIASES_BLOCK+="$ALIAS() { \"${SHIM_DIRECTORY}/$ALIAS\" \"\$@\"; }"$'\n'

• test/artifacts-helper/test_shim_integration.sh
  • Add checks that shell function definitions are present in /etc/bash.bashrc and each appears at the start of its own line