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/request/openai-to-claude.js

@@ -2,6 +2,7 @@ import { register } from "../index.js";
 import { FORMATS } from "../formats.js";
 import { CLAUDE_SYSTEM_PROMPT } from "../../config/appConstants.js";
 import { adjustMaxTokens } from "../helpers/maxTokensHelper.js";
+import { safeParseJSON } from "../helpers/jsonUtil.js";
 
 // Empty prefix matches real Claude Code behavior (no tool name prefix).
 // Previously "proxy_" was used but this is a detectable fingerprint difference.
@@ -281,7 +282,7 @@ function getContentBlocksFromMessage(msg, toolNameMap = new Map()) {
             type: "tool_use",
             id: tc.id,
             name: toolName,
-            input: tryParseJSON(tc.function.arguments)
+            input: safeParseJSON(tc.function.arguments, tc.function.arguments)
           });
         }
       }
@@ -332,16 +333,6 @@ function extractTextContent(content) {
   return "";
 }
 
-// Try parse JSON
-function tryParseJSON(str) {
-  if (typeof str !== "string") return str;
-  try {
-    return JSON.parse(str);
-  } catch {
-    return str;
-  }
-}
-
 // OpenAI -> Claude format for Antigravity (without system prompt modifications)
 function openaiToClaudeRequestForAntigravity(model, body, stream) {
   const result = openaiToClaudeRequest(model, body, stream);