Fix 3 HIGH issues with parameter validation + QA improvements

PARAMETER VALIDATION FIXES (3 functions):
1. lib/common-functions.sh:238 - command_exists()
   - Added [ -z "$1" ] && return 1

2. lib/php-detector.sh:284 - get_fpm_memory_usage()
   - Added [ -z "$1" ] && return 1

3. lib/user-manager.sh:271 - get_interworx_user_domains()
   - Added [ -z "$1" ] && return 1

QA SCRIPT IMPROVEMENTS:
- tools/toolkit-qa-check.sh: Filter out AWK/sed field references
  - Problem: $1 in awk '{print $1}' was detected as bash parameter
  - Solution: grep -v 'awk\|sed' before checking for $1-9
  - Impact: Eliminates 7 false positives from functions with no params

FALSE POSITIVES ELIMINATED:
- is_server_stressed() - $1 was from awk command
- calculate_server_memory_capacity() - $2 was from awk command
- calculate_balanced_memory_allocation() - $2 was from awk command
- list_cpanel_users() - no parameters
- list_interworx_users() - no parameters
- list_system_users() - no parameters
- press_enter() - $1 was from neighboring function

IMPACT:
HIGH issues: 10 → 10 (fixed 3, eliminated 7 FPs, but 10 new remain)
Need to improve QA script further to extract exact function bodies

lib/user-manager.sh

@@ -269,6 +269,7 @@ get_plesk_user_domains() {
 }
 
 get_interworx_user_domains() {
+    [ -z "$1" ] && return 1
     local username="$1"
 
     # Method 1: Use listaccounts.pex to get primary domain