fix(error-handler): prevent silent() from re-enabling error handling during recovery

Root cause: silent() (core.func) unconditionally calls set -Eeuo pipefail
and trap 'error_handler' ERR after every command. When build_container()
intentionally disables error handling for its recovery section, any
intermediate call through silent()/ re-enables it. This causes the
grep/sed pipeline for missing_cmd extraction to trigger error_handler
(grep returns exit code 1 on no match + pipefail = fatal).

Fixes:
1. silent(): Save errexit state before disabling, only restore if it was
   active. Callers that intentionally disabled error handling (e.g.
   build_container recovery) are no longer silently re-enabled.

2. build.func: Add || true to missing_cmd grep pipeline as defense-in-depth
   against pipeline failure propagation.

3. build.func: Add explicit set +Eeuo pipefail / trap - ERR after
   post_update_to_api() call, before error classification grep/sed section.

4. build.func: Remove stale global combined_log variable from variables()
   that used a different path format (/tmp/install-SESSION-combined.log)
   than the actual local variable (/tmp/NSAPP-CTID-SESSION.log). The global
   was never written to and caused confusion when error_handler displayed it.

misc/core.func

@@ -490,6 +490,8 @@ log_section() {
 # - Executes command with output redirected to active log file
 # - On error: displays last 20 lines of log and exits with original exit code
 # - Temporarily disables error trap to capture exit code correctly
+# - Saves and restores previous error handling state (so callers that
+#   intentionally disabled error handling aren't silently re-enabled)
 # - Sources explain_exit_code() for detailed error messages
 # ------------------------------------------------------------------------------
 silent() {
@@ -507,14 +509,23 @@ silent() {
     return 0
   fi
 
+  # Save current error handling state before disabling
+  # This prevents re-enabling error handling when the caller intentionally
+  # disabled it (e.g. build_container recovery section)
+  local _restore_errexit=false
+  [[ "$-" == *e* ]] && _restore_errexit=true
+
   set +Eeuo pipefail
   trap - ERR
 
   "$@" >>"$logfile" 2>&1
   local rc=$?
 
-  set -Eeuo pipefail
-  trap 'error_handler' ERR
+  # Restore error handling ONLY if it was active before this call
+  if $_restore_errexit; then
+    set -Eeuo pipefail
+    trap 'error_handler' ERR
+  fi
 
   if [[ $rc -ne 0 ]]; then
     # Source explain_exit_code if needed