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

tools/toolkit-qa-check.sh

@@ -471,8 +471,11 @@ while read -r file; do
         # Get function name
         func_name=$(echo "$func_line" | sed 's/^\s*//; s/(.*$//')
 
-        # Check if function uses parameters
-        if grep -A 20 "^[[:space:]]*$func_name()" "$file" 2>/dev/null | grep -q '\$[1-9]'; then
+        # Check if function uses parameters (exclude AWK/sed field references)
+        # Get function body and filter out awk/sed commands before checking for $1-9
+        func_body=$(grep -A 20 "^[[:space:]]*$func_name()" "$file" 2>/dev/null | grep -v 'awk\|sed' || true)
+
+        if echo "$func_body" | grep -q '\$[1-9]'; then
             # Skip if uses safe default pattern: ${1:-default}
             if grep -A 5 "^[[:space:]]*$func_name()" "$file" 2>/dev/null | grep -qE '\$\{[1-9]:-'; then
                 continue
@@ -480,7 +483,6 @@ while read -r file; do
 
             # Skip if function doesn't actually use positional params (only uses local vars)
             # Check first 10 lines of function - if all $1-9 are in local declarations only, skip
-            func_body=$(grep -A 10 "^[[:space:]]*$func_name()" "$file" 2>/dev/null)
             if ! echo "$func_body" | grep -v "local.*=" | grep -q '\$[1-9]'; then
                 continue
             fi