Add parameter validation to 6 more functions + QA improvements

PARAMETER VALIDATION FIXES (6 functions):
1. lib/common-functions.sh:219 - format_duration()
2. lib/php-detector.sh:277 - get_fpm_process_count()
3. lib/user-manager.sh:263 - get_plesk_user_domains()
4. modules/performance/hardware-health-check.sh:44 - add_finding()
5. modules/performance/hardware-health-check.sh:55 - command_exists()
6. modules/performance/network-bandwidth-analyzer.sh:45 - add_finding()
7. modules/performance/network-bandwidth-analyzer.sh:56 - command_exists()

All functions now validate required parameters with:
- [ -z "$1" ] && return 1 (single param)
- [ -z "$1" ] || [ -z "$2" ] && return 1 (multiple params)

QA SCRIPT IMPROVEMENTS:
- tools/toolkit-qa-check.sh: Skip $@ / $* passthrough functions
  - Added filter for echo/printf functions using only $@ or $*
  - Example: cecho() { echo -e "$@" }
  - These don't need validation as they passthrough all args

PROGRESS:
- HIGH issues remain at 10 (different ones now)
- Eliminated more false positives
- Next: Fix remaining issues in bot-analyzer.sh

lib/common-functions.sh

@@ -217,6 +217,7 @@ format_bytes() {
 
 # Format seconds to human readable time
 format_duration() {
+    [ -z "$1" ] && return 1
     local seconds=$1
     local days=$((seconds / 86400))
     local hours=$(((seconds % 86400) / 3600))