fix: model cost/info matches first substring key (gpt-4o-mini billed as gpt-4o) (#1439)

* fix: match model name to the longest known key, not the first substring

* test: model key matching prefers the longest specific key

static/js/model/matchKey.js

@@ -0,0 +1,19 @@
+// static/js/model/matchKey.js
+//
+// Pure helper for matching a model name against a set of known keys. No DOM —
+// safe to import anywhere and to unit-test under node.
+
+// Return the most specific (longest) key that is a substring of `name`, or null.
+// Returning the first match instead made "gpt-4o-mini" match the shorter
+// "gpt-4o" key — billing it at gpt-4o rates (~16x) and showing the wrong
+// context window.
+export function matchModelKey(name, keys) {
+  const n = (name || '').toLowerCase();
+  let best = null;
+  for (const key of keys) {
+    if (n.includes(key) && (best === null || key.length > best.length)) {
+      best = key;
+    }
+  }
+  return best;
+}