fix: respect all manifest locations in smoke-test post-install verification (#1952)

The install smoke gate was hardcoding .github/plugin/plugin.json as the
expected manifest path after copilot plugin install, which caused a false
ail for plugins whose manifests live at plugin.json (root) or
.plugins/plugin.json instead of the Copilot CLI convention.

Replace the hardcoded path with a call to the existing indPluginJson()
helper that already probes all three candidate locations in priority order.
Separate the 'install directory missing' check from 'no manifest found' so
error messages surface the actual root cause.

Also fix a .plugin/ → .plugins/ typo in EXTERNAL_PLUGIN_ROOT_MANIFEST_PATHS
(external-plugin-validation.mjs) which caused the error message shown to
submitters to reference a path that indPluginJson never actually checks.
Add cross-reference comments on both constants so they stay in sync.

Closes: reported in issue #1837

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

eng/external-plugin-validation.mjs

@@ -23,10 +23,11 @@ export const EXTERNAL_PLUGIN_POLICIES = Object.freeze({
   }),
 });
 
+// NOTE: Keep in sync with PLUGIN_JSON_CANDIDATES in external-plugin-quality-gates.mjs
 const EXTERNAL_PLUGIN_ROOT_MANIFEST_PATHS = Object.freeze([
   "plugin.json",
   ".github/plugin/plugin.json",
-  ".plugin/plugin.json",
+  ".plugins/plugin.json",
 ]);
 
 function resolvePolicy(policy) {