fix(tools,api): address three additional review findings

- api_v3.py install_plugin_requirements: replace hardcoded plugin-repos
  fallback with config-driven resolution (plugin_system.plugins_directory),
  matching the pattern used elsewhere in the module
- api_v3.py _fix_json_arrays: recurse into converted and existing array
  elements when items.type is object, so nested numeric-keyed dicts inside
  array items are also normalized
- tools.html toolsAction: check r.ok before r.json() and recover
  gracefully from non-JSON error bodies (HTML 500 pages), consistent
  with the existing loadGitInfo guard

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/templates/v3/partials/tools.html

@@ -229,7 +229,14 @@
             headers: {'Content-Type': 'application/json'},
             body: JSON.stringify({action})
         })
-        .then(r => r.json())
+        .then(r => {
+            if (!r.ok) {
+                return r.json()
+                    .then(d => Promise.reject(new Error(d.message || `HTTP ${r.status}`)))
+                    .catch(() => Promise.reject(new Error(`HTTP ${r.status}`)));
+            }
+            return r.json();
+        })
         .then(data => {
             const ok = data.status === 'success';
             showResult(