refactor(open-sse): extract safeParseJSON util, dedup tryParseJSON (B5)

Consolidate two tryParseJSON variants into helpers/jsonUtil.js with explicit
fallback param. Preserve exact per-call semantics: openai-to-claude passthrough
(fallback=str), geminiHelper null. geminiHelper keeps tryParseJSON re-export.
No behavior change; gate: no regression.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/translator/helpers/jsonUtil.js

@@ -0,0 +1,5 @@
+// Safe JSON.parse: non-string passthrough; on parse error return caller-chosen `fallback`.
+export function safeParseJSON(str, fallback) {
+  if (typeof str !== "string") return str;
+  try { return JSON.parse(str); } catch { return fallback; }
+}