Add problem-details: global exception handling with RFC 9457

Companion code for "Global Exception Handling with ProblemDetail (RFC 9457)
in Spring Boot 4". Thirteen failures under five handling setups (Boot
defaults, the Boot flag, a ResponseEntityExceptionHandler advice, advice
plus an ErrorController, a catch-all ordered first), validation errors,
i18n, content negotiation, Security's 401/403, silent 500s and decoding
on the client. 16 tests pin the behaviour.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3TETMrqVUWeFkNtz3Jbo3

problem-details/scripts/demo-client.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# What a RestClient caller can decode from each error body. -> docs/output/client-decoding.txt
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+{
+  for profile in advice ""; do
+    "$MODULE_DIR/scripts/run.sh" "$profile" || exit 1
+    echo "# Profile: ${profile:-defaults}"
+    echo
+    for path in /orders/999 '/orders?limit=500' /orders/boom; do
+      echo "## RestClient GET $path  ->  ex.getResponseBodyAs(ProblemDetail.class)"
+      curl -s -G "$BASE/diag/decode" --data-urlencode "path=$path" | python3 -m json.tool
+      echo
+    done
+  done
+  echo "# ProblemDetail with one extension member, serialised three ways"
+  "$MODULE_DIR/scripts/run.sh" advice || exit 1
+  curl -s "$BASE/diag/mixin" | python3 -m json.tool
+} > "$OUT/client-decoding.txt"
+"$MODULE_DIR/scripts/stop.sh"
+cat "$OUT/client-decoding.txt"