fix: false positive on verify_install script being external shell (#1704)

- Introduced a new test script for shell detection logic in
`verify_install.sh`.
- Updated the `package.json` to include a new test command for shell
detection.
- Enhanced the `verify_install.sh` script to accurately check the
current shell interpreter using `/proc` and fallback methods.

This improves the testing framework and ensures better validation of
shell detection functionality.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- Bug Fixes
- Installer now detects the actual interpreter more robustly and
strictly requires Bash; non-Bash environments produce clear error
messages and abort installation.
- Tests
- Added automated shell-detection tests to validate behavior across
environments.
- Test suite expanded to run the new shell-detection checks as part of
the standard test command.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

plugin/source/dynamix.unraid.net/usr/local/share/dynamix.unraid.net/install/scripts/verify_install.sh

@@ -6,11 +6,19 @@
 check_shell() {
   # This script runs with #!/bin/bash shebang
   # On Unraid, users may configure bash to load other shells through .bashrc
-  # We check if the current process ($$) is actually bash, not another shell
-  # Using $$ is correct here - we need to detect if THIS process is running the expected bash
+  # We need to check if the interpreter running this script is actually bash
+  # Use readlink on /proc to find the actual interpreter, not the script name
   local current_shell
-  current_shell=$(ps -o comm= -p $$)
-  
+
+  # Get the actual interpreter from /proc
+  if [ -e "/proc/$$/exe" ]; then
+    current_shell=$(readlink "/proc/$$/exe")
+  else
+    # Fallback to checking the current process if /proc isn't available
+    # Note: This may return the script name on some systems
+    current_shell=$(ps -o comm= -p $$)
+  fi
+
   # Remove any path and get just the shell name
   current_shell=$(basename "$current_shell")